禁 here means 'forbidden / palace area' (cf.
紫禁城 Forbidden City), not 'prohibit'. Hence
禁军 = 'army of the forbidden quarters' = the troops stationed inside the palace. The Song dynasty famously made
禁军 its main standing army, a system featured prominently in
《水浒传》 (Outlaws of the Marsh) - the hero
林冲 is its drill instructor.
