Luckily for him he was able to resolve the issue on his own and promised to not have any more unapproved house guests, so I didn’t have to evict them both.

If it continues to be a problem then give them the requisite legal number of days to resolve the issue to your satisfaction.

I find that when I come in for a conversation, the problem gets resolved as so many people are passive aggressive and talking takes that away.  90% of my tenants are under 30 and they'd rather text me than ever talk to me directly.I'm a GC and sound proofing the right ay is very expensive to retrofit. 

If they don't understand (which they should) then you pull them in a room with your C.O. and explain that you already signed a lease with an incoming service member, I would imagine things would resolve themselves immediately. 

@Teresa Knepper , I would encourage you to hire a PM after your issue is resolved.

My lease do not specify what they have to do with landscaping, but I tell them if I get any complains from neighbors or the city, then they have to resolve it, or I will hire a landscaper and bill them for it. 

I'm not sure how you'd prove something like that with out something like an arrest or video/photos.Have you even approached them (management) about moving to a different unit as an attempt to resolve this with out having to break the lease?

Many people have provided good ideas and suggestions, but at the end of the day, you need to manage your rentals like a business and stick to agreed upon terms, regardless of your tenants hardships.Many investors have buy rent default insurance to recoup lost rental income from bad tenant outcomes and for certainty in their rental results.Hope you’re able to get this situation resolved soon! 

If you are missing credits for rents owed from the day of closing until when the next rent is due, or if you're missing credits for security deposits, get that resolved before you close.  

Simply tell the co-signer there is a problem with his SS# and until he can get it resolved you will not be able to use him as a co-signer for the applicant.