Nope, they can't proof it was you writing it, did they see you write it? Ok they have an Ip adres then you got loads of options:
It wasn't me behind the computer, everybody uses my computer
There is technology that disguises IP adresses, you got the wrong adres
.......

But the best is yet to come, I can say I smoked 4 joints, but how are they gonna test it? how do you kow its true? could be just one....

Short version: They haven't got enough evidence....