Stopping bad entries in PHP form

I have a PHP form that allows end users to request a sample of the products we sell. Unfortunately, a person/people have found it and are sending in bad requests. We sell only within the US, and so I've set up the form so that they must choose one of the 50 States. But we keep getting requests with countries in the city field, i.e. "Moscow Russia". Is there a way that I can scan for country names, etc. in the text fields and stop a request from going through if it finds one of those "banned" words? I've searched for a solution but haven't been able to find it. If this is not enough info, please let me know. Also, I only know enough PHP just to be dangerous, so please be kind. =;) Thanks, Frank