Minecraft wiki entry for Trap_design, associated text: Don't alter any redstone (either active or inactive) unless you can see the entire circuit. Even inactive redstone can be part of a really long clock.
If you find the method, disable that (take arrows out of a dispenser, take TNT, knock down cacti, take/fill in lava). It may be a two method trap, so you're not necessarily safe yet.
If all else fails, try to safely trigger the trap. Either step on the trigger and run, or use redstone to trigger it from a distance. It is best to have a good understanding of the nature of the trap before attempting to trigger it, as one can easily underestimate its size.
Another way to disarm a trap is to explode it with TNT. This doesn't work with bedrock and obsidian.