I'm going to let some girls sleep in my flatmate's room even after he said no.
Why would you let them sleep in there then?
Well, my plan is to trick him in a nice way without hurting his feelings, and then hang up on him.
It's nice that you want to help these girls by giving them a place to sleep, but you shouldn't do that at the expense of your roommate. If your roommate said no to people sleeping in his room, then you need to respect that. If you disrespect your roommate's personal space, he may choose to move out or simply stay and resent you.
Meh but I want all these girls to come, don't you want to see pretty girls in his room?
Well your flatmate doesn't want them in there, so it doesn't really matter what you want. You should respect his wishes and take them somewhere else.