Our upstairs neighbors have been knocking on our door and running up to their apartment at 2 in the morning and recently there has been gum placed under our door handle.
Missouri does not recognize a claim for "neighbor harassment."
You do have two types of "nuisance" claims, a civil claim and a criminal claim.
The cheapest to handle is the second. Speak with your local police about filing a complaint and they will no doubt speak with your neighbors. It that does not stop it, you will have to develop proof of who exactly is doing this before the police will go further.
To file a civil nuisance complaint, you can use Small Claims Court without an attorney. See the Clerk of the Circuit Court in your area to go that route.
This comment does not create an attorney-client relationship. The law and its application by the courts is constantly evolving and changing. This discussion is not to be taken as a definitive guide, and should not be relied upon to determine all fact situations. Each set of facts must be examined separately with the current case and statutory law analyzed and applied accordingly.