In case you weren't at the faculty conference on Friday here's a solution to the sticky green thumbtack on discussions from Stephanie Dunham (if I have the wrong Stephanie I appologize!), then updated by Heidi Barrett:
The thumbtack will disappear once Vista thinks you have viewed every message. To view each message most easily try going into each topic and "expand all", then "select all". After that choose "create printable view" to see all of the messages.
The other option that seems to work is to open each thread in each unit.