in editor:
r.Lumen.ScreenProbeGather.ShortRangeAO 0
in the default engine ini file:
r.Lumen.ScreenProbeGather.ShortRangeAO=0
There is i think no option to disable the ShortRange AO on spcific objects right now.
You could also try if you using the landscape grass to play around with this command and setting it to 0
r.Lumen.ScreenProbeGather.ScreenTraces.ThicknessScaleWhenNoFallback=0
This will only remove the ambient occlusion on the grass with the grass layer method if you paint the grass with the foliage tool it will still do a bit ambient occlusion on the grass