Unable to hide Dashboard Blocks

I've just been trying to hide some of the dashboard blocks from some specific roles of people as they don't need to see them.  We have a 'Life Member' category, and Life Members are probably inactive members of the chorus, and as such don't need to see things like Tasks if they log in to maintain their details.
I set the block to be visible to the 'Member' and 'Prospective Member' roles only, but when I log in as someone who doesn't have either of those, I still see the block.
I've flushed the caches to see if that was the problem, but it appears that the Dashboard ignores all the standard Drupal rules for visibility and does whatever it wants.
Anyone else noticed this issue?