The Event Category is in use on an Event Calendar part. You will need to delete it from all Event Calendar parts in order to delete it from Code Tables. 
 
1.    Go to Site Explorer
2.    Select Parts
3.    Select Search tab
4.    Select Filter by Type
5.    Select Event Calendar and click Search
6.    Edit the first Event Calendar part on the right
7.    Find the Event Categories section on the part and find the Event Category you need to delete
8.    Click the Black X to delete it from the part
9.    Click save
10.Repeat steps above to delete the Event Category from each Event Calendar part
11.Go to Administration>Code Tables
12.Find the Calendar Event Category Code table and you should now be able to delete it