In the Staff of Magi's description, it is written:
Spell Absorption. While holding the staff, you have advantage on saving throws against spells. In addition, you can use your reaction when another creature casts a spell that targets only you. If you do, the staff absorbs the magic of the spell, canceling its effect and gaining a number of charges equal to the absorbed spell's level. However, if doing so brings the staff's total number of charges above 50, the staff explodes as if you activated its retributive strike (see below).
Does this mean that you just use your reaction and that's it? the spell is canceled?
I'm looking more for a Rule as intended answer because I don't like the idea of my spellcaster player just cancelling my lich's spells.
In worst case scenario I can still just avoid attacking him or bend the rules a little bit, but still.