From Pre-suasion:

Several decades worth of research shows that, in general, the more someone needs our help, the more obligated we feel to provide it, the more guilty we feel if we don’t provide it, and the more likely we are to provide it.

The best way I’ve found to break this is to have rules around what you will and won’t do. Like a project minimum, or only taking calls on a single day a week. Sticking to these rules filters out a bunch of needy clients you’d feel guilty about not helping.