I have a couple of troublesome customers like this, i have been using the envelope thing for a month now and it working pretty well.
I think the thing to do is find work to replace them then if they're that much hassle then drop them.
I know when im full up which will be early next year i have a "hit list" of droppers, only a few but they are just pains
My fav customer who hadnt paid for 3 months
im sure was always in every time finally caught her last month, wasnt going to clean it unless she paid up but when i knocked i heard a kiddie shout and then came to the door 3-4mins later she came, made her excuses then said, "oh i have money today" LOL sort of meaning she didnt have money last times!!!!!!
Andy