What I do in those cases is I keep scrolling through their memes, and click on the ones that have a large amount of comments on it. Chances are, that person will have a few comments responding to people on their meme, so I'll upvote their comments.
Also, if you use the URL to go back far enough in their memes (if you've sorted them by "Latest"), then you can find some that you might not have upvoted yet, if they signed up to imgflip before you did. That's pretty much all I can think of.