Perhaps require that the user have a number of hit dice equal to the caster level of the particular spell card; or have the spell cast at a caster level equal to the user's hit dice, which may potentially result in the spell fizzling if a wizard/cleric/etc would be unable to cast the spell at the same caster level. But the latter would also make them rather powerful as an item that replicates spells, as they generally tend not to scale with the user.