After 12 Months of having to put up with a Jagex chosen name one would hope you have learned a lesson.
If you pick another bad name I'm assuming you would get another 12 Month Jagex name.
If after that you pick yet another I'd assume a banned account. I'm guessing here as I've never heard of anyone coming back after 12 Months using a bad name again.
https://www.jagex.com/en-GB/terms
6. Your account and license.
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.
Hope that helps.