To turn a link into a barrier, simple select the link and set the "Use as Barrier" link property. A barrier link will be display as a thick red line (when selected, a link uses a thin blue line, so you'll have to de-select a link to see the thick red formatting).

When using a barrier it is normal, but not a requirement, to lock the end points so that the barrier remains where you've created it (or moved it).

As a convenience for creating more complicated barriers, gbAnimator provides a "Convert Object to Barrier" toolbar button. This will convert all links, within the currently selected object, to barriers. It will also lock all nodes within the barrier.