A lot of people are suggesting being cordial. For example, "thank you for the request but we cannot handle it at this time". This works for the usual people asking for feature requests but your problem is not so simple.
If you have a persistent requester you can still be professional but far more firm. I would personally say something along the lines of "I have noticed you making a lot of requests. I do not have time to handle them all, but I would be happy to discuss a support contract with you or your company billed at my hourly rate if you need additional support".
Usually demanding money and then ignoring them is enough to either convert them into a customer or get them to buzz off to someone easier to pick on. Since this isn't your case - they won't pay at all - you can be more firm. I don't use github but if it's possible just flat out block them. You could additionally just reaffirm that given the frequency of their requests you would normally service this with some kind of contract and then ignore them.
The key take away is never give someone who exploits you the chance to take advantage of your kindness. It's at these points you need to be firm. Otherwise they will continue to harass you.