I am surprised you've taken such a disappointingly boring shortcut. I had thought I could expect so much more from you.
Your ever popular way of announcing something, assuming it to true is fine when you pontificate on mundane items. In this case, however, I am going to hold you to the more difficult task. If you wish to see the Necromancers closed and eradicated I'm sure you can find the followers willing to invade the city that houses the guild and force the closure (as Genesis has suggested).
Whether the Necromancers fail will not be based on your intimidation tactics. I'm sure you can be more creative.
Written by my hand on the 17th of Hindyear, in the year 1143.