What do I do with a forked repo after I'm done with it?

I’ve forked some repository, made a change, submitted a PR and it was accepted. Now I no longer want the fork. Should I just delete it? … if I do, will I be able to fork it again in the future?

You can leave it and ignore it. Or if having an unused collection is bothering you delete it. Yes you’ll be able to re-fork in the future if necessary.