If you have a OBD2 reader and an app like Torque or Dash command, you can do it that way. Also JB4 and BM3 (possibly MHD??) can at least hide the CEL from showing, but it wont pass emissions.
In terms of a spacer, from my reading it doesn't seem like a simple spacer does a whole lot to trick the o2 sensor.
Give this thread a look:
http://www.2addicts.com/forums/showthread.php?t=1477837
This is a catted o2 spacer that seems like it could work. Since I am getting ready to go catless, I have been following the above thread as a possibility outside of my BM3 tune.
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2015 435i (current): VRSF Catless Downpipe, Bootmod3 Stage 1, xHP
2008 Corvette (sold):Vortech supercharger, ported/built heads, catless
2015 M235i (sold): CTS Turbo Intake, VRSF Catless Downpipe, CSF FMIC, MPE, xHP, Bootmod3 Stage 2 E30