XF 1.5 Inheritance problem


On my forum, I created in a category a forum. And, in this forum, I put a forum called Forum1. (Category - > Forum - > Forum1). The problem is that the permissions of the Forum takes tops on the permissions of forum1. I would like to know how we make to deactivate this inheritance. I have already look in the onode option but the button inherite parent class is to shoot.

Thank you for your help

PS: Sorry for my bad english, I'm french.

(If you can answer me in French, thank you:D)