If you did the designs, charged them design fees, and they paid for them, they have the right to them, just like buying anything else, unless you have an agreement or contract stating otherwise. Quickest way to part ways is send them the files, thank them for their business like a professional, and be done.
If the split wasn't on good terms, and you wanna be a pain, send them in AI format only, not PDF, and number all the files, rather than name them what they are. Most customers for stuff like cornhole boards won't have Adobe, won't be able to view them, know which is which, or what to do with them. If they come back for different file formats, it's a one hour minimum design fee for file conversion, up front, per company policy. Thank them for their business like a professional again, and go buy lunch.