The Collaborative International Dictionary of English v.0.48:
Withholder \With*hold"er\, n. One who withholds. [1913 Webster]WordNet (r) 3.0 (2006):
withholder n 1: a person who refrains from granting; "a withholder of payments" [ant: granter] 2: a person who restrains or checks or holds back