I suppose the question is: does the SEPM think these clients are online? Does the icon have a little green dot on it in the SEPM console?
If the client record on the SEPM is online, then deleting them from the console will be temporary. The client record will be recreated the next time it checks into the SEPM (where it will then take up a license and update from the SEPM or as configured by the LU Policy).
If the client record on the SEPM is offline (and has been for some time), then it should drop off the SEPM automatically anyway (after a default 30 days).