Make the switch also activate some water (a current passing glitched water will make it flow normally) to wash away part of the redstone, stopping further current.

This Link from Minecraft wiki shall guide you through a huge number of simple traps. These are designed to trap or trick other players or mobs automatically. Some traps even kill...

However, I shall briefly sum up the most popular designs for trapping spiders.

Land Mine For a simple land mine a 1 by 1 hole two blocks deep. Next put one piece of TNT at the bottom and cover it with a block that looks natural to the area(a stone block in a cave ect.). After that just put a pressure plate on top and it's done. If a mob or player steps on it, It will go off. Note: When using a wooden pressure plates dropped items will trigger the TNT and the delay and noise of the TNT may alert the victim.

More Advanced I pay attention to Kiershar's youtube channel, and based my mob trap off of his videos. The biggest problem with his grinder as well as many other traps is that it does not kill spiders very efficiently, and sometimes, not at all. I've seen videos of elaborate fixes to this problem, but I can't believe someone hasn't seemed to have done what I've done.

Here is my grinder. Just like Kiershar's, I have a 3X2 lava pool running over a couple of ladders with a 1X2 space below where items but not mobs can fall through. The sand block at the bottom left is where a cactus would sit if this were Kiershar's trap. As you can see, I have omitted this as well as a block below the ladders for keeping spiders from falling through because this is not where they will be dying (note the string floating by).

This is how they die:

Here is the start of that same channel that has the lava at the end. The beginning of the channel should have another channel one block higher flowing into it and a cactus sitting on top of that sand block. The length of the channel after that doesn't matter, but this part is crucial. This creates a 2 block high, one block wide space that all other mobs can go through, but not spiders. Taking advantage of the fact that spiders are the only 2 block wide mob is what makes this work.

Here is another view of the beginning of this channel with the cactus placed and the other channel filled in. A water block should be placed to the right of the cactus.

Placing that water block leads to a water current that looks like this. 1 block wide mobs will fall in and most likely float behind the cactus, but soon after will float to the lava and die. Spiders however, will sit here unless there is another mob behind them. They will block the entire 2 block wide entrance to this channel, so they must die first, which they will because the mob behind them will push them directly into the cactus at the 2 block high level. This always works, and no other mobs will flow through until the spider dies.

Here's the spider killer in action. As you can see, the spider floats to the block at the level of the cactus and dies there. This doesn't depend on whether the spider is jumping; it's just somehow levitating above thin air. The ceiling should be filled in at the block above the cactus, though, to make sure the spider stops here.

Spiders will also kill each other this way. No more spiders clogging your trap! Taken from here.