You probably shouldn't just completely ignore it; ignoring may have the potential to cease, but it has equal potential to continue on.
Look at who they are trying to appeal to. Maybe you are subconsciously thinking thousands of people are seeing that, when maybe 5 or 10 people are reading it. If they are not appealing to many people then ignoring would be a good way to go. If not then persuading the target that the accuser is wrong, say if they are making comments on their forum, so you should join and make a strong, professional statement. If you get banned, it will only make their forum look worse, so you need not worry there.
If things do really escalate, whether ignoring or intervening, you may be able to contact the host on the case of libel (similar to slander), and that this illegal activity could also get the host into trouble if the libelous content remains.