Say it straight: 'I will not add this feature' or 'I will only add it if you implement it', whatever is the case for you. You could add: thanks for your suggest

Say it straight: 'I will not add this feature' or 'I will only add it if you implement it', whatever is the case for you. You could add: thanks for your suggestion or something like that. That's it. You may give a reason if you want, but don't engage in argument. Just ghost or block them if they persist. Don't have to fight or yell, just let the other deal with their own disappointment. If they misbehave on your project, ban them. You don't owe anybody anything.