Its obvious that these players have invested heavily in charge based builds. If its frustrating to you and the other players you have to start putting them in situations where they can't charge, or where its less effective. Here are some suggestions:
It sounds like they are moving away from a lot of creatures. Are they provoking AoO? Make sure you take every one that they grant.
Find monsters with auras. When they enter the aura they are slowed. If they have already moved 2 squares they are SOL. Another thought is to find a monster or two who can restrain your characters (spiders are great for this). This will lock them into a place for a turn or two while they attempt to escape.
Difficult terrain is something to consider as well. This will slow the characters to half their speed.
Traps, monsters that mark and monster with reach are other good ways to deal with these players.
Swarms. Pain in the ass, but characters who can perform area and blast powers are especially effective, characters who perform melee attacks not so much.
If your chargers can only target AC think about adding some higher AC lower NAD monsters. (Maybe even run some to hit percentages and find a sweet spot where they are hard to hit for your chargers and easier for your character who target Ref, Will or Fort).
Ranged critters (archers, other artillery monsters) can be quite useful if you put them up in some trees or up on a cliff while the players are in a valley. If the party composition includes some ranged PCs then this is a nice effective way to give them something to target while the chargers are taking care of the ground forces.
But remember, that these players have invested heavily in their charge builds, its what they primarily do and if you take that out of the game to too much of an extent then their carefully constructed characters won't be as useful. Show them that the same trick doesn't work every time, but also don't let it become so heavily penalized that they regret their builds.
Finally look to design your encounters so that everyone has something to do. If you have a wizard who specialized in area affects look for lots of minions. If you have 2 charge characters find something that ties them up. If you have ranged characters give the other side something they can target most effectively.
It depends on what triggered the shift.
As @ioanwigmore pointed out, PHB1 p.287 describes a Charge as (emphasis mine):
Move and Attack: Move your speed as part of a charge and make a melee basic attack or a bull rush at the end of your move.
As @Ravn pointed out, PHB1 pg 268 describes an Immediate Interrupt as (emphasis mine):
An immediate interrupt lets you jump in when a certain trigger condition arises, acting
before the trigger resolves. If an interrupt invalidates a triggering action, that action is lost. For example, an enemy makes a melee attack against you, but you use a power that lets you shift away as an immediate interrupt. If your enemy can no longer reach you, the enemy’s attack action is lost.
Case 1:
If the attack was the trigger (for example, Trigger: You are the target of a melee attack.), then movement is already done. The charger has already transitioned from the "movement" phase of the charge to the "attack" phase, and can't move any more.
Case 2:
If the movement was the trigger (for example, Trigger: A foe enters a square adjacent to you.) then the shift happens before the "movement" phase of the charge resolves (i.e. finishes). In that case the charger can continue as long as they have movement left and each square of movement brings them closer to the target's new location.
The pseudo-momentum doesn't say you can't change directions, only that you must always be getting closer to your target. For example, because of how 4e counts diagonals, the following 90 degree turn in mid-charge is perfectly valid:
oTooooo
ooFoooo
oooPooo
ooooPoo
XXXooPo
ooooPoo
oooPooo
ooSoooo
o = empty space,
X = obstacles,
T = target,
S = charger's starting position,
F = charger's final position,
P = charger's path
Best Answer
Charge is not broken
A character's turn ends on a charge (unless they use a Free Action after the charge), so the best they can do is maneuver then charge. Given that every square of the charge must take them closer to the enemy and that they must move at least two squares, the movement is quite useful here.
The real balancing factor is the melee basic attack. Characters, therefore, cannot use encounters or dailies (unless otherwise specified) on a charge. Charging is certainly one of the better mechanisms a melee striker has for engaging a target of her choice, but is not "broken." Even the most optimized charger is not broken compared to someone who can trivially perform multi-attacks.
As one of the characteristics of a striker is the ability to engage a target of their choice, charge-focused characters enjoy this mechanism for choosing their target at the cost of a great restriction of their at-wills.
My recommendation is to set your preconceptions of older editions aside and treat 4e as a completely novel game. It has its own difficulties, but a simple charge is not one of them. Combat in 4e is supposed to be significantly more fluid than that of earlier editions and this movement philosophy reflects that.