I have a customer that I see 3 maybe 4 times a year, thick pile, cream synthetic carpets from top to bottom in a town house, I went back last week and the carpets were dirty than ever before, tank filled with foam, then mr customer said "she got it in her head to buy one of them carpet doctor things, cleans them every other week, they take days to dry and get dirty again really fast, I explained what the problem was and he said right that's going in the bin then and we'll carry on as before.
It's best to let them learn by they're own mistakes, and they'l come running back to you when they realize you were right all along, bit like children really