It can be a bit brutal, but they might actually appreciate it.
You definitely do not owe someone who's harassing you a gentle rejection.
Plus, this is a pretty important part of the emotional connection between two people.
Explain the reason You don't actually owe an explanation of real reasons for your refusal.
In fact, it takes a girl a lot of courage to decline a date.
You can slowly share some photos of yourself with someone and people will make their own assumptions from it.