overvote

Short definition from Wiktionary

Noun

1. 1
   A case in which a voter has marked a ballot for more choices than allowed by the rules
2. 2
   A case in which more votes are cast in a voter's name than allowed under the rules

Verb

1. 1
   To spoil a ballot by marking it for more choices than allowed by the rules
2. 2
   To cast more votes than allowed under the rules
3. 3
   To outvote or overrule