[qpurser]

I am not a pro in coding but did something similar. Not exactly the same what you are looking for but it might help to figure it out.
I didn't wanted to hide a block but display a message instead in that block to animate the unregistered/visitors (usergroupid 1) to register.

my code:

<vb:if condition="is_member_of($bbuserinfo, 1)">
Only Registered/Logged in users can view this section
<vb:else />
**** add here the contents you want to display and remember to add "</vb:if>" at the end of the contens you display****

by doing this everybody not belonging to usergroupid 1 will see the contents of the block