Multiple Workflow Instances

I had a similar issue, I have a workflow that sends email notifications. To ensure multiple emails weren't sent when someone made multiple changes to the same View, for each control with a rule to add a notification activity I created a hidden data label and I updated the rule that runs when the control is changed so that it runs only if that hidden data label is empty and the last part of the rule is data transfer to populate the hidden data label.

I still have an issue with clearing a hidden Data Label I use to combine workflow notifications. For example, I have a hidden Data Label (hdnViewNotifications) for each view on my form that combines all of the notifications that might be triggered on that View any time the form is filled out. To add a notification's name to the hdnViewNotifications control I use the Transfer Data and add the hdnViewNotification control and that specific notification's name with a comma after the activity, so that the hdnViewNotification combines all of the View's notifications in that single control (see attachment). Then when the form is saved a rule pulls each View's hdnViewNotification control to add all of the appropriate notifications to the workflow.

Sometimes users change a control so it adds the notification's name to the hdnViewNotifications and then reset the control. If a control that triggers a notification is reset, I would like to remove that specific notification's name from the hdnViewNotifications. Can anyone direct me on how to "clear" a specific notification name from the hdnViewNotifications control without clearing any other notification names that might have also been added to the control? Do I have to repopulate the hidden control with an expression created to remove the specific notification's name? If so, any pointers on how to build the expression would be greatly appreciated as I'm not very comfortable with expressions.