I'm not sure if this is what you are after, but the simplest way to stop the math adjustments from showing up on the cards for your attack powers etc. is to simply equip the non-magical equivalent of the item, making sure to uncheck the boxes on the Equipment page in the 'Show on Power Cards' column for the magical versions.
That is, if they are to have a Magic Dagger of Whateverness, give them that magical dagger but equip the non-magical 'Dagger' item and ensure that only the 'Dagger' has a tick in the checkbox for 'Show on Power Cards'.
EDIT: The 'Show on Power Cards' option is found in the sixth column of the Inventory tab, which is the default first tab of the Equipment section. It may not show up unless you scroll a bit sideways, and only equippable items have checkboxes.
The items cards will still show their bonuses, but only on the actual items cards themselves - the power cards will show only the inherent bonuses. All special powers granted by the magical version will show up on the item cards themselves. The one exception are those items that grant class-related powers, such as the Tome of Summoning granting extra spells. The extra spells will not show up unless you equip the item, which forces the adjustments to show up on the power cards again. This can be worked around by using the 'House-rule' function on the 'Powers' page to simply manually grant the addtional powers separately from the items.