General Antonyms:
* Abandon: To give up completely.
* Neglect: To fail to care for or attend to.
* Destroy: To cause damage or ruin.
* Disrupt: To interrupt or break up.
* Disregard: To ignore or pay no attention to.
Specific Antonyms:
* Decline: To refuse or reject. (For "maintain a position" or "maintain an argument")
* Reduce: To lessen or decrease. (For "maintain a level" or "maintain a standard")
* Withdraw: To take back or remove. (For "maintain a presence")
* Release: To let go of or set free. (For "maintain a grip")
The best antonym for "maintain" will depend on the specific situation.