I think I'd likely tackle it as follows:
First, create a folder where the topics can be moved. (I'd err on the side of caution, moving them first to a folder before wholesale deleting them)
Then, from the Topic List pod, I'd right-click the heading area and enable seeing the Build Tags. Then drag it into view. That should allow you to sort topics by build tags.
From there, select all the topics containing the build tag and drag them to the folder. Then behind RoboHelp's back, copy the folder to a different folder on your hard drive for safekeeping for our dear friend Justin.
Keep in mind that removing the undesired topics from the RoboHelp Server will likely be a separate process. However, as long as you delete them from RoboHelp, the Source Control issue shouldn't require any special actions.