When setting up an Account for a Jagex Product, you may be asked to choose a username that will be used to identify you to other users. For example, a Jagex Product character name. You must not choose a username that infringes the rights of any third party, impersonates Jagex staff or other users, which is deliberately confusing, offensive, racist, obscene, hurtful, unlawful or otherwise inappropriate or which breaches the username requirements specified in the applicable rules relating to the Jagex Product. We reserve the right (at our sole and absolute discretion), to change any username for any reason or take such other action as we believe appropriate.
Source:- https://www.jagex.com/en-GB/terms
Threads locked. Nothing can be done if Jagex forced changed your name because you could not pick a good user name after several warnings.