A better approach would be to use a FeatureWriter to delete these features before they get written. You can add a WHERE Clause to filter out those you want to remove

The problem is that the pre and post SQL does not have any notion of feature attributes, so you cannot reference any feature attributes in the SQL. However, you can reference private or public parameters.

If you need to delete based on a feature attribute, there is the DatabaseDeleter transformer, or you can look into the suggestion from @hkingsbury​ 

@hkingsbury and @david_r, thanks for the advice. I will try use where clause. Basically delete existing records before inserting new ones. Use this for RDBM like Oracle all the time, never used against a file geodatabase.