I believe you will find there already is a system for Justice in place in
every city. It is the Ministry of Justice, and if you feel that a crime hahs
been committed within city walls (and as the guild is inside the city, the
guildhouse most likely counts), then it is up to you to take the matter to
your respective Minister of Justice.
If your Minister of Justice is halfway competent, he/she and his/her aides
will investigate the matter, and deal with the miscreants. Or, if the
Ministry has a different approach, then it will behave accordingly. At any
rate, that's the system you should turn to for this matter.
Lady Alecto D'Malachia, High Priestess of Justice
Written by my hand on the 11th of Springflower, in the year 1020.