Can I disable or remove Ultimate Vault Hunter Mode?
At the BL2 menu, just select New Game to start over with a new character.
Or if you want to keep playing your original character in order to finish any missions you missed, do a Save-Quit to get back to the main menu, then press a button (Y on the Xbox 360) to bring up a list of characters you have played, even if it's just the current character. When you select the current character from the list, your next option is to play the starting level or True Vault Hunter.