If you feel like you physically being threatened, report to the police immediately, otherwise not much you can do besides blocking or any other legal means

13

As #1 said, police first if you feel threatened. They can often contact companies to skip a lot of the shenanagins, if it’s a real harassment issue. Unfortunately, the odds of finding out much information about someone are slim to none because if they’re using mostly 3rd party services to harass you. Snap/yak/instagram ect, the messages hit the company server before they get to you, so you never see anything directly from anyone like you would an email.

1