Multiple choice is ok. If somebody decides to vote for everybody, then their vote won't actually make any difference. On the other hand, a person may think any of three candidates out of six would be ok as a moderator, so they could vote for all three.
I like the idea of a "none of the above" option, because it might turn out that no suitable candidate nominates for the position.
Another idea would be to have two rounds of voting - the first one multiple choice to whittle the field down to 2 or 3 candidates, and the second one single choice among the three candidates.