I like Faith's graduated plan. Have the scratch off available for new people with a pop up screen at the end (win or lose) that directs people to the forum to learn how to make more money. I would give them a couple of months of that.

After the "newbie" period, have the game closed unless there is other activity that quarter (shopping, survey or offer).

In the 2 and half years I've been a member I think I've only made $5 with the scratch game. Can't imagine just coming for that.