I have a blocking device on my phone where if they call more than once, they get the BIG block..If you don't have it here are some options.. I like answering their questions with questions.
1. after answering, leave your phone line open they can't disconnect: open lines are fun, Put the phone next to a running vacuum cleaner, a loud music speaker using really annoying music, try a garbage disposal, let some kids talk on the phone.
2. Call your law enforcement & report harassment or phone stalking (which are both against the law) you already have their number.