Originally Posted by Sly

Give them a warning that you will need to disable services if payments are not made by X date.

For all future clients, make it very, VERY clear that this is your policy and you do not fuck around.

The moment you allow someone to walk all over you is the moment they realize they can, and they will. If their website is vital to their business, which I'm guessing it is, they will learn to understand that paying that bill is akin to buying groceries.