Guildmistress Inshallah has my full and unreserved backing on this serious matter. The Thieves Guild will no longer tolerate any unauthorised stealing (or any other activity which is in their domain).
As the patron of the Guild I will not hesitate to use all means at my disposal (and believe me, they are considerable) to help track down and bring to book any transgressors.
You have had your warning. There will be no more. Only punishment.
Mephisto, god of the night, Patron of the Thieves Guild