Some magic items have counter magical properties. A Dispel scroll or Destroy Magic scroll are the most obvious examples. If a player has the option of using such a device he must declare this when the spell is cast.
A Dispel from a magic item is always a flat dice roll or, in some cases, automatic. It is not modified by the caster's level and power cards cannot be used to boost the Dispel or to reinforce the spell.
Some magic items confer a Dispel specifically against spells directed upon the model carrying the magic item or the unit he is with. For example, a spell cast against a unit which carries Mork's War Banner is dispelled on the D6 roll of 4+. These kinds of Dispel work in the same way as a Natural Dispel as explained below.