There's no reason to feel guilty. Because:
Think of yourself as being a friendly guide to a hapless exchange student (pun semi-intended). They're unfamiliar with the culture. Even if their rep isn't that low, they could still be unfamiliar with the system (their rep could've gotten artificial boosts by registering for multiple SE sites). Some of them will probably even thank you for the reminder. Which leads us until reason 2...
You are actually helping the OP and the community by encouraging good habits. I think the site would be a lot worse off if no answerer ever reminded posters to accept. Reminding posters can actually be done quite unselfishly. A non-accepter is actually harmed because potential answerers may avoid posters with low accept rates.
Accepting costs nothing (except a negligible amount of time) and is the way posters should show gratitude. If they can type "thanks" they can click a check mark - uninformed users should be instructed that the two actions are synonymous. If they've already thanked you for the answer, you have good reason to believe they would immediately oblige upon being reminded of the proper etiquette.
You're hiding behind the anonymity of the Internet, dummy!
Watch your tone (don't call people dummy). Say it like you're trying to help said hapless exchange student (it helps if that's at least part of your motive, which hopefully it is). I usually say "please" and inform that accepting encourages others to increase the quality of their answers (the implication being that people like to be rewarded). Saying things like, "hey, where's my accept? I thought you said my solution worked for you! I'm never going to answer your questions again." ... is probably not going to help. I would venture to guess that answerers with the nasty latter attitude don't have much reputation to show for it.