If the service you are providing is to host the site and he isn't paying you, then you are within your rights to take the site offline (within the terms of the contract).
Given you technically own the domain I suppose you could go ahead and use it, but it would be at best a grey area and could potentially cause some bad PR, legal and trust issues with other clients. I'd seek proper legal advice on that one tbh.
Shut the site down if hosting isn't paid. Seems he doesn't care about the site.
You definately can't take the site material. You might want to ask him if he wants to give it to you to pay off what he owes.
I should explain that the content on this small 3 page site was done my me and a former partner. He had some content that he had wanted put up and gave it to this former partner - who lost it all in a move across country. The graphics are work that I did.
In any case, I would not use the current content.
Still... it has been marketed as his site so I do think I'm probably skating on perilous ground here, even using the domain name without his agreement.
Thanks for your comments. It confirms what I was thinking.
Also consider why he is "refusing" to pay.
Cant he pay, or wont he pay?
If he cant pay, i'd suggest restrict his service somewhat but keep him on board so he has time to pay. Try to agree a final payment date, or smaller instalment payments with him.
If he wont pay, if he has a legitimate complaint against you, deal with that amicably.
If he wont pay and there is no complaint about your service, then I'd remove his site immediately and move on to more productive business.
Send him an email or letter telling him you've done that and why - in some cases this results in immediate payment and they'll take you more seriously. Otherwise he's taking you for a ride.
If he won't pay (and if there's no legitimate reason for this) then one option would be to force him to pay through the small claims court (or at least, to threaten to).
This does of course completely screw any future business relationship.
You're also entitled to interest, if he goes beyond his payment terms - if you're in the UK, google for 'bank of england late payment rate' to see the applicable interest rate (which is quite high), assuming this isn't specified in your contract.
DELINQUENT ACCOUNT page usually takes care of this
If you own the domain name it is ours to do with as you deam fit. Show me a phone company, electric company or water company that doesn't disconnect your service after 30days of non-payment. What is your company name "Free Ride Here"?
If you think you can do something with the site and domain I would do the following...
1. Remove the site from the server until he pays.
2. Keep the domain registered.
3. Send him a certified letter telling him what you have done and that you intend on taking over the domain in X days for non-payment.
I would then wait one month and send him another certified letter and follow up email that you now own the domain and will be deciding what to do with it. Finally I would wait 3 full months after sending the last letter and email to see what happens. If he never contacts you and never pays anything I would do whatever you want with it.
However as a safety pre-caution I would not use his content even if you developed it. I would simply use the domain name and re-invent the content doing what you want to do with it.
If you give him all the notice above and plenty of time to react and he never does then I think you are within your rights to do what you want. After all if he were dealing with a registrar and hosting company they would pull his site within a few days of non-payment in most cases and the domain would go back to the public sector in approximately 60 days. You are simply not letting what would normally occur for non-payment because you don't want to take a chance to losing the domain which I believe is reasonable.