Some of the people talking about it were adamant that they would do it. Even if they weren't serious how is he meant to know? I thought they sounded pretty serious.
Boarding up the windows suggests he is afraid of more than just stickybeakers. It wouldn't suprise me if he had recieved threats or hate mail.
From a reliable source (so take at will): at the wake a guest said loudly, within earshot of GBC that if GBC same near him (the guest) that he's deck the murdering *advertiser censored*.
Same source has said that GBC knows people think it was him (including ABC's folks) and that people have said things to him - so I'm not at all surprised he's worried.
At the end of the day, even if he's being over protective, it's better than being under protective.