Regardless of whether the guest is genuine, it is an important question and should be answered for all of us.

The statement that one bill was paid late is a statement of fact, of which the truth could be established.

However, the allegation that the person does not pay bills in general is not justified by one example, and is (potentially) libellous.

If you have no luck with the site owner, the ISP hosting the site should be approached, as they may also be liable for damages if they do not remove libellous material on request. (Of course, it may be an inhouse site).

I think even ebay would remove material if it risked being sued for libel. The law will take precedence over company policy!

As noted above, depending where you and the company are located, various other legislation may be applicable, and of more use than the libel laws. UK Data Protection act violations are criminal, not civil, offences.