Spell shop problem!

Hello there, i'm making a map that gives the players the option to buy spells, unfortunately i have this problem balancing it. so what i've done so far, is making a shop for each spell category, like: "Passive Spells", "AOE Spells" And so on, but my final problem is that i want to make it so, that the player can only buy one spell from each category, but i have no idea how to this.

Help would be highly appreciated!

It is difficult to discuss this without knowing how your spell buying works. Show us the spell buying triggers you have so far (right-click on trigger name in the main trigger view window -> copy as text -> paste it on the forum using [trigger] tags) and we can help you figure out what to add in order to limit spells to one per category.