I found a number of questions on Meta relating to revenge downvoting, but most of them were discussing the situation in which a user suddenly receives a deluge of malicious downvotes.
My situation is different — a single, tit-for-tat downvote in direct response to me downvoting their post, which the other user explicitly admitted in a response to my comment:
me: [explains why their post didn't adequately answer the question]
them: [correctly inferring that I downvoted them] "I downvoted your question too then."
I won't lose too much sleep about the -2 rep, but this is clearly not how downvotes are supposed to be used. What's the best response to this situation, or should I simply ignore it?