It is a spell not a ward.
I am not that much more proficient in magic rules than you

, but, I can think of three approaches.
1. cirucumventing the spell's trigger. It's safe to assume, it will not fire on himself, or on the beings that he marks with his signature, so if you're able you can do some masking
2. you can trigger it by disturbing the anchoring - casting a mana spell on it in astral should be enough, if it is however a mana spell it might be set to fire on astral disturbances so you might be affected
3. you can dispell it using a standard counterspelling on sustained spells (anchoring however gets a bonus from the karma to resist such attempts)
In the cases 2 and 3 the caster will know if the spell was dispelled or triggered. In the case 1 he will not.
advisable read. pg. 59 of street magic (anchoring)