You can write in big letters "We're volunteers working on this project for free, don't request things, ask nicely", have a notice to ask people to never write us in private, more notices about "if you don't pay, we probably won't prioritize you" and "we don't have unlimited time, if you really want something fixed, fix it yourself or pay someone to fix it" and STILL people will ignore all the notices.

After a couple of years of politely declining, it gets tiring. It's OK to ignore people. It's not rude at all! I care more about my own free time (that I spend writing open source software) than I care about being polite to people ignoring instructions and expecting us to work for free FOR THEM.