Sadly, You can’t block IP addresses on Squarespace according to this https://support.squarespace.com/hc/en-us/articles/217549458-Activity-Log-Analytics
but I’m having a similar issue and wish there could be a way.
If it’s any consolation, from what I’ve read about blocking IP addresses, it would be very easy to go around. So it doesn’t sound like it would be worth it anyway.
Just doing a search on internet harassment I found some websites that might help:
https://onlineharassmentfieldmanual.pen.org/
https://www.digitalinformationworld.com/2019/03/infographic-how-to-handle-online-harassment.html?m=1
https://privacyrights.org/consumer-guides/online-harassment-cyberstalking