That is it on or off, no other user definable settings.

The Quick and Full system scans both do a rootkit scan, but of a lessor degree of sensitivity.

Why disable the scan (you would then lose that protection against a legit alert), it is no real hassle to just click OK to the default action Ignore in this case and allow the avast CommunityIQ function to report these suspicions and be analysed and hopefully corrected quickly.