If you are qualified to become a paladin, you will become a paladin during the ending cutscene, there is no way to get the Paladin ceremony and refuse the honor or choose not to become one. The same way you can't get both the good ending and the bad ending in most video games, you can't get the paladin ceremony and stay in your original character class, it's one or the other. It's been a long time since I played the game however so I could be wrong.
Alternatively, you can become a paladin, but when it's time to import your character in the third game, choose to import it as a fighter or wizard instead of a paladin.
Now, if you don't want to become a paladin there are some actions you can perform during the game that will disqualify you from becoming one:
- Taking part in any break-ins side quest if you have thief's skills
- Finishing off the opponent during the Eternal Order of Fighters initiation
- Performing the infamous X-Ray specs easter egg
- Taking money from Omar's purse before returning the purse
- Keeping Rakeesh's sword until Uhura reminds you to give it back
- Killing Khaveen's while he's unarmed
- Slaying the griffin
The first six actions listed above result in an automatic disqualification, and slaying the griffin, while not being an automatic disqualification is a technical disqualification because in order to become a paladin, you must have collected at least 25 paladin points out of a possible 34 paladin points prior to the paladin ceremony, and disturbing/slaying the griffin gives you a -10 point penalty, and so, even if you did your best to become a paladin after disturbing/slaying the griffin, you would at best collect 24 points, one point short from becoming a paladin.
I would also argue that, in order to become a paladin, one must forget about "having done everything I can do" because the griffin feels a lot like a mini boss, at least in the remake, and whenever I play that game and don't care about becoming a paladin, I always have a ball fighting the griffin.