you lot will get a wake up call one day. 30 days is nothing. removing someone's site b'cos they have not paid within 30 days is, imo, childish.
there is no law that states you must pay within 30 days. jsf, its a contract that is entered into between yourself and your client, be that a verbal agreement, an email or the t&c's stated on your invoice. don't think that just because you put 'payment within 30 days' on your invoice that anyone will pay that any attention - you need to agree conditions like that before you start any work and if money is that much of an issue to you, then ask for half up front.
get with the real world.
in this case i would re-invoice them and add 8% and send it by registered post. if you don't hear from them within 2 weeks then take it all down - don't make rash decisions, all businesses go through quiet patches and you don't want to lose it all. remember that word of mouth recomendations are the best way for you to get new business.