Pixelmon Mod

How do i disable it cause if i leave the area the trainer despawn.

Use a "Trainer Editor" and place the trainers yourself, those will be permanent.
The ones you seem to be refering to are trainers that spawn like any other Pixelmon, and therefore has a despawn timer.

You get the editor using the Creative menu.

You can disable the spawn of the random generic trainers in the pixelmon.cfg and then just place the ones you want using the editor