Make it like awesomereminders.com and people get called and bitched at until they finish their project...
"Did you do anything about it yet?" "Did you do anything about it yet?" "Did you do anything about it yet?"
I bet getting a call every day would work extremely well, bringing the same dread as a call from a bill collector with a far more straightforward path to mitigation.
At 1 call per minute (which I figure is pretty liberal for a "Hey, have you finished your shit yet?") a single person can clear 400 people per day easily.
But of course if you paid someone $10/hr to call people, your sweet spot for break even is somewhere around 350 customers before you hire someone.