The console command is just:
I bound it to a key so I could quick veto votes.
If you want to be fancy and make the command look like it came from the server instead you do:
And finally because SM automatically hooks onto console commands you could just say any of the following in chat:
Whatever you do you want to be quick. Veto doesn't actually override the vote, it just sets all remaining ballots to No which may not actually be enough to stop the vote. If anyone has any ideas on implementing a proper vote veto let me know but the voting mechanism seems to be built in with limited access.