@ Mr Santa Man - There is nothing that can be done from the forums in regards to the player that messaged you. That needs to be reported in-game at the time of the incident, as that is the only way to provide Jagex with the evidence they need from their own systems. All chat is reportable, whether its in public, private, clan or a friends chat.
What you can do is provide Jagex with the information you have using the instructions and the 'Report Incident' button in the 'I lost my items due to a hijacker' section of the
Lost Items article
in the Support Centre.
You should also make sure that you are taking advantage of all the account security tools available, such as a Bank Pin and RuneScape Authenticator
Bank PIN
RuneScape Authenticator
If you need additional security advice, you are welcome to ask in the
Community Led Account Help Forum.