Deleting Shared Sub-string Names Follow

Example:

- I create 2 text fields. Each containing one sub-string of Screen Data.
- I name one sub-string "Counter1", and one of them "Counter2".
- I check my sub-string list in the Data Sources Toolbox, and see that there are, as to be expected, two named sub-strings in the list.
- I rename each of the two sub-strings to "Counter3" and "Counter4" respectively.
- I check my named sub-string list again, and now there are [b]four[/b] named sub-strings. The two I just named, and the previous two that should have been over-written.

How do I get rid of these no-longer used named sub-strings, so that I can re-use the names with new sub-strings? And to avoid clutter within my list of available sub-strings to link to VB scripts?

Thanks