No Exp or leveling, ever.

You can try to alter the $C0/9F78 function. The function calculates the starting level when a character is created.

As example, the following code will force the starting level of any character to be 50:

Code:

org     $C09F78

    LDA #$32        ;set start-up level to #$32 (50)
    RTS             ;return level

The characters still will have their level alteration as setup in their startup data. You can easily setup them for zero alteration with ff3usme.

Indirectly, you can edit the HP/MP growth table and the additional HP/MP in the startup data to setup a fixed value for the character HP/MP.

I must also say that without level up some abilities won't be updated. Specifically, Sabin's Blitz, Cyan's SwdTech and Celes and Terra natural learning. They are dependent of level alterations to be updated.