you cannot prevent RACH..
can you verify that those RACH are caused by Location Updates ?
in those cells you could increase the T3212, so that the faulty MS would not do the periodic LU so often.
During daytime, those MS are originating or terminating calls, so the LU timer T3212 does not expire.
My assumptions is that the faulty behaviour occurs only during the expiry of the t3212 (periodic location update)…
however, your problem is “virtual” : nobody will ever complain that there is sdcch congestion during the night. It doesn’t impact anyone.
Also, you could try to activate the immediation assignment rejection, that should help.