After playing a nice sacrificial game against my opponent, he accused me of cheating. I played that game honestly and didn't use any outside assistance of any sort. In fact, the only reason I even knew to sacrifice a piece in that position was due to it having arisen in a previous game of mine. What should I do?

I had the same experience: a guy say to me that I cheat for eating his pawn "en passant"...

You have three options:

1. Ignore it

2. Block them

3. Report them for being abusive: https://support.chess.com/article/209-how-do-i-report-someone

It's up to you.

You will not be banned if you are playing clean, no matter how many people report you.  Probably pretty much anyone who plays actively on the site has been reported at least once, and if you've played a large number of games the fair play team has almost certainly already investigated at least some of your games.

Option 4 prove them wrong by asking them to a rematch

You can simply ignore any accusations from your opponent. It means nothing, except that he's a poor loser.  Don't even bother to respond or engage with them, it's a complete waste of time

If you weren't cheating, you won't get banned.

Nobody gets banned because some sore loser accuses them. Don't sweat it