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It depends on how you make the hair. If you make the hair in the way, that every hair is its own body, then you just delete the ones you don't need, before you merge the rest of the hair with main body and the wall. But making every hair as its own body, can really slow down your software. It depends on how much hair you make.
Another option is, that when you make the hair with pattern, you only join it together with the wall and not the main body. Then you cut the hair that you don't need with extruded cut or revolved cut or similar. Then you join it with the main body.
If these instructions arn't clear enough, I might make some sample object (similar to christmas tree) over the weekend, to demonstrate.