Since they are driving your crazy, simply and firmly tell them

simply and firmly tell them that you dont' renew this house. no need to answer why. No responding for their writings are more powerful than argueing with them or explain anything. Sometime ignore is very powerful to handle something like this.

If they mention about email what you have agreed, then tell them that you have spent hundred hours for this house renting, this is unexpected at the time. This is a business for you, you can spend this much time for one house. For those people, simple , firm and ignore are important.

This is my suggestions,

1) Write them a formal  letter, and mail them using certified mail, standard mail and email to notice them the lease temination after this lease .  The standard mail and email will add the certified mail' certification number as the mail header. Do this in 60 days or follow the contract
2) No explanation, just tell him that you have different plan.
3) If they complain, writing you long letter, don't' reply, but if they want you repair, do it asap and using licensed contractors. If they request items which don't' need to repair, charge them trip fees, give them invoice on those trip fee.

Hope this can help.