You can type disengage to stop fighting whoever you're fighting, and you'll stop attacking whether or not they're fighting you. If they're fighting you, you'll keep defending, until you either do something aggressive (reinitiating your attack), flee, or they type disengage.
Any agressive move will stop your disengage. (kick, bash, disarm, sap, backstab, charge, hit, kill)
See help disengage for syntax and details.