How to block someone when the option to block them is not letting you do so!

Have a situation, someone started talking to me on iMessage but thought it was a someone I knew but it was all their information and picture but it was a hacked account. Tried blocking them but the option to do so keeps putting me off and not letting me ! What can be done? I don’t want them hacking my information, I haven’t click on any links that they have sent and they are NOT friends on my FB account. Any help would be greatly appreciated.