And to be completely clear, there's actually another step -
Mouse over "family trees," click on "more," then click on "trees shared with me" to see the list of trees to which you have accepted invitations and click "remove from list" to any and all.

You can also leave them in that list and just stop receiving emails about changes/additions - mouse over your username and click on "my alerts" to display "Family Tree Alerts." From that list select "Change delivery options" for the tree(s) in which you are no longer so interested.