I don't think saying something like, 'I don't date clients' is rude or mean. Then you get the boring, lovestick-lamb bleating to change your mind.
I guess you still want their money but don't want to piss them off.
But he's imposing and nagging you isn't he?
Personally, I'd ignore, block and ban as it's likely some skewed obsession for romance or free sex and who needs the drama! But that's me.