Banishing is severing the bond between a spirit and its summoner. When the bond is broken, the spirit returns to its own plane. This is not the same as disrupting a spirit and forcing it back to their home plane—while it interrupts the spirit at what it’s doing and may be annoying, banishing is not as downright unpleasant as disruption (and a spirit engaged in a tedious task might not mind being banished in the least).
Banishing is a Complex Action. You make an Opposed Banishing + Magic [Astral] vs Force (+ Summoners Magic if Bound) Test (you can spend reagents to set the limit of this test, p. 316). This test is opposed by the spirit’s Force (+ the summoner’s Magic if the spirit is bound). For every net hit you get, the number of services the spirit owes is reduced by 1. If you reduce the spirit’s owed services to zero, it’s free and departs on its next action.
The Drain Value for banishing is equal to twice the hits (not net hits) on the spirit’s defense test, with a minimum Drain Value of 2. If the spirit’s Force is greater than your Magic rating, the Drain is Physical, otherwise it’s Stun.If you (or another magician) has an action before the spirit departs, you can use Summoning (p. 300) to try to get it to owe you some services. It doesn’t matter what type the spirit is or which tradition you are in this case, since it’s already out and available.