I realize that this post is almost a year old so if you have figured this out already or if the question is not longer relevant, I apologize. Here is what I would do. Create a variable that captures the name of the current configuration. During the For Each Configuration portion, create a Jump/Label action that compares the configuration name variable to "@" so if the current configuration name does not = "@", remove the number value. If the current configuration name is "@", then it should jump to a label bypassing the "Set variable value" action.
Would that work for what you are doing?
Derek M. Lawson