Quite difficult, the easiest option which you don't want to hear is to change them.
Have they actually done this or are you expecting them to do it? Have you suddenly started receiving those things out of the blue? Could it be unrelated?
If you are certain it is them you could advise them that you are going to go to the police as I would say this is a form of harassment (and any sites they've registered on may have a record of the IP address used which could possibly be traced to them).
Or... you could in theory I suspect take legal action of some sort - perhaps small claims court for damages to pay for a new phone and email to avoid the situation they created.
Of course you would need some kind of proof.
Easier to resolve it amicably or just change the things.
Worth talking to your phone provider and getting some spam filtering software. Your phone provider may be able to block content of some kinds.