If Marowak's "Bodyguard" ability is in effect, preventing the effects of attacks on me and my hand, does Greninja's "Shadow Stitching" attack turn off abilities of Pokemon in my hand or not?
Shadow Stitching turns off the Bodyguard ability, so until the end of your next turn none of your Pokemon will have Abilities.
Source: TPCi Rules Team (2016-05-05)

Relevant cards

Note: All applicable cards may not be listed below.
XY10_37
XY9_40