I want to display a message to users in our ticket form when they select certain options. I was hoping for a "message box" type of field but I'm not seeing that, so I am interested in other possible options.

Ideally I would like the behavior to be: user selects option from drop-down list >  message is displayed > user cannot submit a ticket unless they change options. 

One answer might be to just not include an option that I don't want them to use, but it would be more helpful to include the option with an explanation.