Apparently, it is a foul to obstruct or hinder a player and his flick, although it almost seems impossible to police without any clarification.
This has been a problem for me in a few games, not being able to make a block flick with a figure because of obstruction ( unable to see the play, or unable to make the play )
I think it might be a good idea to introduce an 'obstruction' call, suspending play to allow the defender time to take a blocking flick after the attacking flick, only in cases where both players are on the same side of the table, and one is bossing the space.
To be honest, I think there is a conduct of sorts in the Old Subbuteo community, where players do not seek to gain advantage through somebody else's disadvantage. Not intentionally, that is. But it might be an idea to have something in type.
What do 'the people' think?