Theoretical seat blocking can always be removed without any warning or notification. The seat blocks also don't appear on cabin crew heldheld devices so of they need to reseat a passenger they'll see it as an open seat. I'd therefore always be extremely cautious about playing the seat blocked card.... it may well backfire on you. Ultimately the only surefire way to prevent an adjacent seat being snapped up by an interloper is to purchase it as a comfort seat. Why you'd want to nab the entire row in Club is beyond me.... theoretical seating for the entire row is just ever so greedy and completely unnecessary. I appreciate it's not you theoretically blocking the entire row off but

I'll admit to passively aggressively protecting the middle blocked seat on an exit row in Traveller on more than once occasion however. This was when I was sat in the A seat and the C seat passenger appeared to be gesturing for a traveling companion to make the move to come and join us.

Man spreading over the middle seat with the armrest retracted, placing personal items on the middle seat and feigning sleep until after pushback and again after take off have all helped me out previously.