This probably isn't the most efficient way, but you could use a variable to store EP, and have one common event control the levelling of it, and have another which has a series of events which look like this:
Conditional Branch: Armour [Iron Armour] Equipped
Conditional Branch: Variable [001: EP] < 3
Play SE: Wrong
Text: You do not have enough EP to equip this piece of armour. You need 3 and you only have \v[1] EP.
Change Equipment: Armour [None]
Branch END
Branch END
It's not very clean, as I am not sure but it will probably wait until you exit the menu before delivering the message and unequipping. It'd be easier if you can get a script, but that should work if all else fails.