Anything that makes the user invest a bit in the reputation of their account before they are allowed to message strangers would do the trick. Charge $10 for signing up. Limit @-mentions until you have built up a small following and activity track record. Require an application to join the forum. Etc, etc.
If a cost is evenly moderately costly to create, and abusive comments quickly result in bans, then the ratio of abusers quickly goes down to a very low level.
The problem is that making it costly to sign up goes against the business model and growth model of sites like Twitter, Reddit, or Instagram.
If a cost is evenly moderately costly to create, and abusive comments quickly result in bans, then the ratio of abusers quickly goes down to a very low level.
The problem is that making it costly to sign up goes against the business model and growth model of sites like Twitter, Reddit, or Instagram.