Here are the steps to my problem:

1. I double click a cell in the bill of material to edit a part's custom properties.

2. After editing the cell or when I leave the cell without editing it, a letter and number appear (example: B1, B2).

This number and letter that appear puts this value in my part's custom property at the part level:  

Then I have to manual go to the part and remove this string of text everytime I edit a part custom property within the bill of material on my drawing sheet.

How do I stop this from happening?