How To prevent skill stacking

How do i prevent a powerup skill such as protect (though im not going to call it that...) anyways, how do i make it so that once you use it in battle you cant stack the effect, meaning you cant keep using it over and over and over and keep getting +10 def each time, how do i make it so using it will only give the effect once? becouse if it did it all the time it would be kinda cheep...

I was not aware that it stacked to begin with 

thats weird. mabye i should re install it or somthing. oh well. nevermind that i guess.

As my knowledge provides, it's not possible, mostly because in rpg maker 2003 increase of defense or any stat is like recovering hp, all it is doing is adding to a number, only difference is, it only lasts for that battle...

and if someone manages to find a way to stop that then I'd be learning something new  ;;