How do you formally inform a prospective tenant that you don't want to lease the apartment to them?.

.- 06/18: I emailed her to send new lease draft and ask to formally renew, saying "let us know by 06/20".

You'd need to formally evict them through the courts, which takes months to do. 

How formal or informal does it have to be or is it a simple, I would like to buy your house lol.

We lend...but I've also got a formal credit background from two of the largest banks in the world and 32 yeard of this under my belt.

Proceed with extreme caution.Regarding comps: you could always pay for a Broker's Opinion (BO) or formal appraisal.

You will typically (but not always) get a formal hearing, but you don't have to physically go as long as you have your ducks in a row (i.e. have compelling evidence provided to them in advance making your case for the valuation dispute).

It was a CFD (we have clean title), its been vacant for a few months and we're selling without a judicial foreclosure / ejectment. 

Then, when you DO catch them, or find actual evidence of a particular violation, follow through with the consequences in a formal, businesslike manner by giving written notice with deadlines as allowed by local law.

We had another instance where we texted him and on the 3rd I gave him a formal notice.He just went out of town for 2 weeks and we went to check on the unit and found he has vandalized the unit.