I don't think it should be banned. People often have abusive phone calls. Sometimes (though less often now with the internet) people get abusive or threatening letters. But no-one suggests we should get rid of phones or the post because of this (or individual postal/phone companies etc).
There is already safeguarding against people abusing or threatening others - it's called the law. Anyone breaking the law can have their information passed to police by the site, even if it's not visible to the public.
It is quite easy to report a crime online or over the phone, however, though I don't think it's necessary I wouldn't object to a link added to websites such as Ask.fm (something like 'What to do if you're worried or upset about a comment you receive.') which simply gives information about people to talk to (samaritans, childline etc) as well as how to report crimes in different local areas and what is not acceptable. I think most people of the age who should be using these kinds of sites should be able to find this information themselves, however, it can't hurt to make it even easier to find and could help.
xxx