If I'm not interested: I just don't answer or block.
If I have to ask for a picture of a guy and am not interested after seeing what he looks like: 95% of the time I'll just stop there. 5% of the time (if I feel bad for the guy) I'll say something like "thanks" and then never respond again.
If I'm still having a conversation with a guy and slowly become disinterested in him: Usually there's a point in the conversation where I get a good reason to duck out of it. Like if he lives with other people or doesn't drive etc.
There was one guy I dated and after 2 semi-awkward dates lost interest in him, and in that situation we had like one mini text conversation and afterwards I just stopped talking to him completely. Then blocked him lul
The times I have been honest with a guy usually do not go well. They will make a nasty comment about your appearance or personality or something.
On grindr I'm one of the better looking guys. It's rare that I get someone who says to me, "Not interested", but when I do I just assume it's someone
A.) Fugly (usually they are faceless + insecure),
B.) Fake, or
C.) Someone I've blocked/turned down before in the past who remembers me doing it to them
