A suggestion has been added to cap the level of potion that can be added to an item. From issue #16

Is there a way to cap the level for enchant items with Potion Effects? For example... if I attempt to enchant a tool or armor with 'Strength IV' but the cap is set at 2, you'll be unable to apply the effect or it would downgrade to Strength II.

Should be achievable by interrogating the incoming Potion NBT tag and manipulating level