Put simply, you can't. Warnings are permanent and will only be removed under one of the two circumstances:
1) The warning was INVALID. This means that the warning was for something you did not do/say. Nothing else is acceptable as 'invalid'. If the warning IS INVALID, you must appeal this on the appeal section of the website: http://www.team-neo.com NOT in this forum.
2) You donate specifically for the "warning reset" option on the donation page.
You will NOT be unwarned for any other circumstance no matter how long or what the crime was.