How do I disable build triggering with two VCS roots?

I have a build with two roots:

$/One_Project
  which has /Source

and

$/Another_Project
  which has /Source.

On the build agent, this maps to

Agent Directory
  One
     Source
  Another
     Source

I would like to trigger builds off of anything from Another_Project and ignore changes from anything in One_Project.

I tried adding this rule to the Build Trigger Rules section:
-:=One/Source

But it didn't work.  How can I add a rule to make this happen?