How can I make an potion effect that prevents further poison effect from being applied while active?

if the "selected effect" something that you put there and its not from minecraft, go in its "when effect applied", check for the first effect and if its there, remove the selected effect

and if not, go to your effect's tick condition (because it runs every tick, if you dont want to make your active tick condition always true to run this every tick) and check for the selected effect. if its there, remove it