You could try changing your 2nd choice vote to be your 3rd choice.
Because you can only have one 3rd choice this will remove the vote from the original person. You'll then have cast two votes - your 1st and 3rd. Then change the vote on your real 2nd choice from 3 to 2.
So you start with:
- Person A - 1st choice
- Person B - 2nd choice
- Person C - 3rd choice
You don't want to vote for Person C at all. Change your vote for Person B from 2nd to 3rd. This gives you:
- Person A - 1st choice
- Person B - 3rd choice
- Person C - no vote
Change your vote for Person B back to 2nd. This gives you:
- Person A - 1st choice
- Person B - 2nd choice
- Person C - no vote
This should work.
In fact this works for removing your 2nd choice as well. Do the above and then change Person B to 1st and then Person A back to 1st. The only thing you can't do is remove all your votes this way. You have to end up with at least one vote cast.