What you need to do in this case is find what tables are using the full text index:
In the database that uses the full text catalog run:
SELECT name, ftcatid FROM sysobjects WHERE ftcatid > 0
That will give you a list of the tables which use it, then you can mark each table not to use full text:
EXEC sp_fulltext_table 'tbl_whichtable', 'drop'
Then eventually again try:
DROP FULLTEXT CATALOG catalog_name
http://www.aspdeveloper.net/tiki-index.php?page=SqlTipsFullTextError