vitiate

To reduce the value or quality of; impair or spoil.

To corrupt morally; debase: synonym: corrupt.

To make ineffective (a contract or legal stipulation, for example); invalidate.

To render vicious, faulty, or imperfect; injure the quality or substance of; cause to be defective; impair; spoil; corrupt.

To cause to fail of effect, either in whole or in part; render invalid or of no effect; destroy the validity or binding force of, as of a legal instrument or a transaction; divest of legal value or authority; invalidate.