Sending them to collections puts the monies owned on all 3 credit reporting agencies which is intended to put pressure on them to pay their bill AND warn any other potential creditors of their past.

This is the ONLY warning you will receive and this is not up for discussion.

I can look back and question my decision to go with him based on a number of warning signs, but we know some of the same people and our trusted friend referred him to us.

And before quip "well no, but where have you been around the world" I warn, that would be a long, long list.  

Anyway, good luck and fair warning to anyone considering an str - plug this potential into your underwriting.

Red flags to be aware of: Are there any deal-breakers or warning signs I should be looking out for in the listings or during the initial research phase?

Just providing some warning

Now I get monthly warnings and fines if the trash can is left out even a minute past time, etc. 

That is where you make the move to evict or warn then evict.

Write the tenant a warning.