Rather then using an impact, start with a breaker bar. See if that helps.
As qouted from Doug's Domain:
6.Remove the center shaft retaining bolt with a 16mm socket. A 5/8" socket also fit nicely and can be used if your toolset exhibits an anti-metric bias. If this bolt has never been removed (unlikely for all E36's at this point), you may find it necessary to apply considerable torque to remove it. Mine came off easily because the wheel has been off at least twice.