If the hosting was a separate deal to the design, and the hosting part of the deal has been paid for, then it could be YOU who ends up in court if you then turn off the hosting/mail facilities when it is the design bill that is outstanding.
I have many clients that I both host and design for.
Providing the client's hosting payments are up to date I could NOT legally ever turn off their site for late payment of design fees, as this would be illegal, and immoral.
Trick is NEVER lose your cool with a client - regardless of how they push you. Word of mouth spreads very fast normally anyway, but spreads a lot faster when it is bad words about your company, and it can take years to undo the damage to your reputation that this would cause.
As for getting late payers to pay up, if you are in Europe the normal period for settlement is 30 days from date of invoice. After that time you can consider a payment late, and you are legally then entitled to charge statutory interest on the outstanding balance. This allows you to charge 8% PLUS the current Bank of England rate, so currently you can charge 12.5%
When you inform the client that you will be charging this rate (and these rates should form part of your T.O.S. and be posted clearly on your site and on any contracts you have with clients) they will usually cough up, as that sort of rate makes it much more expensive to "borrow" from you when they are juggling their bills, than to borrow from their bank or use their overdraft facility.
Many companies do juggle bills this way - imagine if you have 3 major outstanding bills, but only enough cash at hand to pay two of them - which do you pay? You pay the two that will cost you the most if they are late!
So make sure that next time they juggle bills they know that being late with yours would be too expensive.
And as for the client posting in the forum - well, providing that the comments are not true, then you need to take action. You need to inform the client that you consider the post/s libellous, you also need to inform the forum owners that you consider the post libellous and will sue if it is not withdrawn.
But whatever you do - do NOT reply to the post in the forum. You do not need to be seen to be doing your dirty laundry in public. Remember that anything posted in a forum can come back and bite you YEARS later, as search engines DO trawl forums and DO have long memories.
Finally, to help avoid problem clients in future, if you are in Europe there are a number of places where you can get credit reports on prospective clients. Costs less that £20 a time, but can save a lot more in the long term.
At the same time you are waiting for the credit report to arrive, do some serious searching on the client's company and personal names - looking for things such as forum posts where he/she/they badmouth suppliers - if you find such things then you know they make a habit of it and can steer clear.