If you click on an enemy unit and target them with your archers the archers will follow the enemy unit to keep it in range.

If this is the case the easiest why to stop it is to hit backspace when they start to wander. This will take the archers out of their direct attack mode and put them back into "fire at will" mode.

The only other thing that would cause your units to wander is if you have 2 of your units too close to one another. In such a case they will move to find breathing room, (but only a very short distance from there original position).