If you mean machine the hub center down to be able to mount he BBS wheels flush with the spacer, then yes you can. (I take it that the hub ring is larger than the BBS hub bore?) Theoretically it helps center the wheel and keeps balancing/vibration down. Actually, if you mount the wheels correctly, cross pattern tightening, then hub rings I don't see as totally necessary.
However, I don't recommend altering anyone's design without careful thought nor am I endorsing you do. Designs, even seemingly simple ones, are that way for a reason. That small lip may actually be thought into the engineering and provide more strength to the slug especially seeing that the mounting holes are in close proximity to those holes. This lip may provide sufficient strength against stress fracture from occurring but when removed it may weaken the part just enough.
You should be fine but still yet a word to the wise that if you fluck it up and wheels go flying off then it's on your shoulders.