Train them.
I tell them at the outset that this is a monthly/two monthly service and that I do not accept on-the-spot cancellations.
Any attempt at a doorstep cancellation is met with a reminder of my terms and a request for an explanation please.
Answers:
Type 1
Don't need doing - dump
Going on holiday next week - dump
The dog goes mad - dump
I'm not feeling well - dump
Type 2
I'm ever so sorry I forgot to ring you, the window frames are being painted tomorrow/builders arriving on Tuesday etc - strike one if they've not done this before and she looks like Cameron Diaz or hands out drinks and biccies - strike two if she's a munter who rarely gives you time of day
Dump if there is no apology from a "demanding" customer.
Type 3
Death in household - a free pass this time.