So, just a couple of baseline definitions. JP gain is static. It can increase as job level and character level increase, but the amount per action is always the same regardless of the action as long as it was successful. XP gain is based upon level difference between the character acting and its target. Acting upon a character of equal level grants you 10xp. Acting on a character lower level than you will decrease that amount.
Another fun fact is that all members of the team gain 1/3rd of the JP of another character's actions. So if a Black Mage does an action and gains 30JP, all the other characters get the JP Spillover amount of 10JP.
So from this information I believe you have two options aside from the throwaway thief.
Get your characters to around level 20 very early on in the game (I am talking after the first story line mission). Then prep for a long battle to raise as much JP as is needed to master a class for the next storyline mission. The enemies in the story do not scale with your level like random encounters. You can use beneficial abilities on your enemies so you do not kill them but gain just a minimal amount of XP per action
Your other option is to do nothing with your character. Put into the group the classes you want to raise. Protect the person whom you want to raise the skills of and then have that character do nothing. Have the others do the simple Double JP + Fundamentals->Focus abilities to gain max JP every time and let the other character gain 33% of that XP. Now granted this will slow your JP gain down to 1/3rd per other team member in the match, but the XP increase is 0 for a character that does nothing but wait.
I honestly think the XP thief is probably the best way to go, but these are the two options I am aware of that may get you what you need as well.