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.

I'm sorry to report that the barrier features in place have some limitations. If node speeds go too high the barrier will not stop them. This is a problem with the algorithm I use and I hope to fix it along with implementing the full barrier solution later this month.