I couldn't agree with this more. Extremely frustrating.
I did find this support info which may help (though it's definitely not substitute for having user ability to manage by any means), but hope it helps! According to this support info, images DO finally get deleted in time (between 8-38 days) apparently, but in an urgent situation you an send an email for SS for assistance in getting them deleted sooner: https://support.squarespace.com/hc/en-us/articles/205812748-Image-and-file-URLs-in-Squarespace
However, Squarespace, PLEASE give the ability to manage this on the user end.