league-of-legends – Skills in League of Legends that Can Hit Targets Outside Their Range: Detailed List

There is a hotkey H which is the equivalent of toggling "Stop", which stops your current action. I'm not sure whether this will do what you need in stopping the auto-attack, but it's worth a try. I don't see any other options that could remotely do what you desire, other than getting stunned.

Continually pacing back and forth within range is a preferable course of action once you learn how to do it without getting exhausted. This actually has a few benefits besides just preventing you from attacking - it makes you a harder target, and it also makes it less detectable to be seen creeping closer and closer towards your opponent to make a pot shot. Alternatively, it makes your opponent uneasy since you can easily be in range to take said shot, which makes them less inclined to stay close.

First, let's clear up a misconception.

Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.

It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.

That said, let's take a look at why Armor Penetration is preferred, even with its linearity.

Attack Speed is easy to acquire via items.

Armor Penetration is not.

You can more than match the effects of attack speed runes with a single dagger - 420 gold.

The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.

It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.

Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.