I think Trump would be detrimental to our country. Although this may have no impact; voting for someone or cancelling another vote, I would feel more justified if I was able to make Trump have one less vote. Either way, I am influencing the election. In this manner, I'd feel like I am doing something that I feel strongly for.
If it was possible to use a vote to vote against someone, instead of for someone, I would use it to vote against Trump. This would be a good idea for people who do not want Trump to be president, but who do not care who else actually is president. If we could vote against Trump, we could avoid splitting our votes among the remaining candidates, and pool our votes against Trump.
I would definitely consider using a vote against someone but only in a primary, not in the general election because I would not want to waste a vote. I would only do it against a frighteningly racist, misogynistic, xenophobic, sexist opponent to keep them away from the White House. We have to keep each other safe in the U.S. so we are our brothers keeper.
Voting against a political candidate is, in theory, the same as voting for another candidate if there are only 2 candidates. To avoid bringing current politics into it, I will call them X and Y. If I really want X to win and do not agree with Y's philosophy, then I can either vote for X (+1) or vote against Y (-1). If there are only 2 people to vote for, then this is the same thing. Let's not make voting even more of a negative by having citizens vote against candidates.