WordNet sense 1 (a blemish made by dirt):

blot, daub, slur, smear, smudge, spot

WordNet sense 2 (smear so as to make dirty or stained):

besmirch

WordNet sense 3 (charge falsely or with malicious intent; attack the good name and reputation of someone):

calumniate, defame, denigrate, slander, sully, asperse

WordNet sense 4 (an act that brings discredit to the person who does it):

stain