While you may be looking at a commercial transaction, that always doesn't apply to private individuals so check the usury laws and see how they may apply.Usury is an unlawful rate of interest.

Once the days have passed, you can go to the small claims court and file for an unlawful detainer.

You should run that report independently, and also search for legal actions (evictions & unlawful detainers), and any criminal convictions.As a former property manager to a few large apartment communities, I tell you that 90% of problems come from poor tenant screening.  

If you really don’t want them there any more, and they truly refuse to leave, you’d be forced to evict them (unlawful detainer action) to remove them. 

Also planning to serve 30-day notice to quit to the unlawful co-tenant on Dec. 31st.  

Since your previous tenant has abandoned the property and this new person is a complete stranger, with no lease, it sounds like an unlawful detainer case which I've found is quicker than an eviction.

It's not worth the risk of unlawful offset.Having said that, congratulations on having (extremely) long-term tenants and on their departure.

The complaints grew and I had no choice but to file an "Unlawful Detainer" eviction.Since then things have become worse.

Tell him that he has put you at serious risk by using your property in an unlawful manner with drug use on the premises and the discharge of a weapon. 

If she doesn't leave, then evict her for unlawful detainer.