Oftentimes, especially if your character is using a ranged weapon, simply clicking on the ground doesn't stop the PC, they just step to the side and keep right on shooting.
To really stop fighting, look at the upper left-hand corner of your game screen. You will see the "attack" symbol, a dagger inside a square. Click on that symbol and your character will stop fighting.
For the mages out there, I don't know about you, but my character always wants to cast Wierd when autoattack kicks in. If you have fast enough reflexes, you can stop whatever casting your PC has started by clicking on the symbol for that spell up in the left-hand corner.
If you don't have fast reflexes, practice apologizing.
