You can move your votes around at will. But you cannot remove the last vote altogether. So, once you cast one or more votes, you will always have at least one vote cast.
It has been pointed out that you can indeed erase all but one of your votes using the following method:
You can only apply one of your votes at a time to any singular one individual. If, for whatever reason, you want to cancel your secondary or tertiary votes (rather than move them to someone else), vote for one of the candidates you do want to vote for as that slot, then reassert the correct slot.
For example, if you voted A as 1, and B as 2, to simply cancel 2, vote A as 2 then as 1. This will remove your 2nd vote until you return to cast it anew.