I think the rule is 80/20. 80% of complaints from 20% of customers. It applies to so many things like 20% of your carpet will get 80% of the wear and tear etc.
It is a great question and one I have come across myself so I smiled a bit when I saw your question. I have done it the good way and the bad way.
The bad way. I wrote to the customer and told them that when their hosting expired in 6 weeks time that they will need to find an alternate host. This was because the level of support and assistance they required was outside the service levels we were offering and we could no longer support their website. The fuss, the to-ing and fro-ing and the general hassle that resulted was immeasurable.
The good way. With another customer I wrote to them to say that we are no longer offering pure hosting services, and that our full annual management fee is £xxx (some generally horribly high figure). As your business does not require this level of support or assistance, we would recommend this host (name some cheap alternative) that will match your needs for a fraction of the cost. We are sorry to be losing you but, once you have opened an account we will gladly FTP your files and database contents to your new host. To take advantage of this simply email us with your new account details and we will transfer all your data for free. Alternatively you are welcome to continue with our services at the higher price.
If they stay, they pay. If they go, you spend 30 mins transferring their data for them and you are free. Believe me, that you will spend a lot longer than 30mins dealing with a 'go away' type letter, and they will leave thinking 'what a nice company, helping us out like that and saving us money'.
Good luck with it though, whatever you do, do not tell them it is because they are a pain.