What can you do about a parent that lives in a nasty and uncleaned house but refuses to leave?

Stop visiting. I rarely go into my mom's home anymore. I go into the living room (the door opens right up into it) but I cannot force her to clean, or hire someone to clean, but one thing does work: her kids won't visit her. She will clean up, somewhat, if she wants visitors.

If your parent is of sound mind and the house is deemed messy but not a danger to her, she can stay as long as she likes.
In extreme cases where the parent does not allow any outside help and the condition of the home poses a danger, the health dept. and/or adult potective services will intervene.
Will she/he allow you to hire a housekeeper or in-home caregiver to come by once a week?