How do I get rid of a person who moved into my dad's house and now taking advantage of him?

Did you happen to take photos of the place at it’s worst? A picture is worth a thousand words.

Call APS and tell them what you have posted here.

Best of luck to you and your father.

Call APS. At 88 is Dad showing signs of Dementia? Do you have POA to take over if Dad is incompetent.

If Dad is competent, then he has to agree that he wants this person out. If so, you can start the eviction process. Hope you have taken pictures, a video would be best. Has the person paid Dad rent? If you don't have POA this is the time to get it. At Dads age, I would also go with an Immediate POA. This will give you some control right away.

You don’t need a cleaning team, you need to get this person out of your dad’s house. Call police or adult protective services.

Following on Alva's good advice, you might also ask the local PD to do a wellness check.  When they see the living conditions, they may take action to condemn the house.   The local county authorities could also be contacted; I believe they would be in charge of condemning the house as opposed to the police, but this may depend on jurisdictional authority.

Is it your father who's become "nasty" or the freeloader?   If the former, APS could get involved, and perhaps take action to eventually remove the freeloader.  If it's the freeloader who's nasty to you, his actions and inactions are jeopardizing your efforts to address your father's health.   That's also an issue for APS, since it could be considered elder abuse.

You might also call the IL state offices to determine what resources are available.  I would think a state the size of Chicago would have some elder law resources.    And the state might even have more support than local authorities.

What is your Dad's role here? Is your father suffering from dementia? If not this may be his choice, and will be difficult to prevent.
I would involve local authorities if you believe your father is not quite competent and believe that this person is taking advantage of him. Call APS in your area.