How do i get someone's twitter username, they haven't been on it for 3 YEARS?

[IceCoolFashion]

I wanted to change my username and the user i want someone already has. but they haven't been on it since December 2010, and it is now 2014... so how do i get their username. i know its possible because someone did that to my older twitter account, so i know it can be done, just no one has told me HOW to yet. and how do i get their account deactivated, i mean seriously! 3 years and twitter hasn't deactivated them yet!!!
micksmixxx actually YOU CAN! i said in the question description that i had a twitter account and i had forgot the email and password i used, and next thing i know someone has my old username! so i think i had got deactivated by twitter and then the username was available again. im just wondering how to deactivate someone cause the account obviously hasnt been used in 3 freaking years! geez get your facts right BEFORE saying something (you clearly know nothing about) is impossible.