There are times when you might come across a tenant who is considered a 'hoarder.' This is actually considered a mental illness and is linked to obsessive compulsive disorder and is covered, in part, by the Fair Housing laws, but that doesn't mean that you have no hope of making sure the tenant complies with safety regulations and the lease that he or she signed.
When a tenant suffers from the symptoms of a hoarder, they may have things piled to such a degree in the home that it actually may be considered unsafe. Your first step should be to take a look and to document everything. Pictures will be a great help in this. Take as many as you can. Give your tenant a notice as to what needs to be done to make sure that the property is not in violation of any codes (if their actions have caused this). If the clutter is so severe that one or two people may never get it all taken care of, you might want to provide them with a company that can help. In the end, though, if they cannot or will not fix the situation, you may be forced to begin the eviction process.