lol, realized I could set the modifier to 0 if capital is in Iberia, or if it's a specific tag, so I did that like this:

modifier = {
factor = 0
OR = {
tag = POR
tag = CAS
tag = SPA
}
capital_scope = {
region = iberia_region
}
}

it should prevent Iberians from choosing exploration ideas in all circumstances, and by proxy should prevent them from choosing Expansion ideas as well.