If "complement" means "to complete" or "to make whole":
* Contradict: To say or do something that is the opposite of what someone else has said or done.
* Oppose: To disagree with or fight against something.
* Defy: To refuse to obey or accept something.
* Incomplete: Not complete or whole.
* Lacking: Missing something necessary.
* Insufficient: Not enough.
If "complement" means "to enhance" or "to go well with":
* Clash: To disagree or conflict with.
* Contradict: To say or do something that is the opposite of what someone else has said or done.
* Clash: To disagree or conflict with.
* Diminish: To make something smaller or less important.
* Degrade: To lower the quality or value of something.
* Mar: To spoil the beauty or perfection of something.
If "complement" means "a thing that completes or enhances something else":
* Opposite: Something that is completely different.
* Counterpart: Something that has a similar function but is different in some way.
* Inverse: Something that is the opposite of something else.
* Antagonist: Someone or something that opposes another person or thing.
The best antonym for "complement" will depend on the specific situation.