On every question that I answer where a user has less than a 100 rep I tell them to accept or optionally up vote my answer. If I don't do this, they might up vote me but if they have less than 15 rep it'll tell them they can't vote and might do nothing at all.
I'm certain you can run reports on first time users to see their behavior compared to users with more than 100 rep.
Lastly, people do not even know there is a checkbox there or what it does (refer to the screenshot). You could have a giant hover box or some sort of message that they should accept the post if it's their first time voting. Please enhance your UI and educate new users better since it is not obvious to them.
This may be a dupe but I'm okay with that. There has to be a better way to teach users so people like me don't clutter the site with pointless 'please accept an answer' comments.
And another screenshot: