Welcome to the real world.

Not every customer wants to pay for some reason... hehehe, even though they know that they need to pay, but they just decided to ignore that fact.
However, you need to be frank with them that you shall suspend their account if they don't pay. And do it! I see a lot of companies out there just threat the client and never folllow through. Bad business. Make sure you mean what you say, and vice versa, if you say that you'll provide great service, make sure you do it! If not, then don't say it at all! It's always better to do more than what is promised.