Thanks for your reply.
I'm using a view as the data source of the report and all tables in that view are linked. (is that what you mean?)
To delete the fields, I opened the report with the older version of the Crystal (the one it's been created in) but as you said I still don't have access to those fields (Alike the screenshot above, there's no Sql Expression Field item on the column). 
Is there anyway I can solve the problem? (Beside creating a new report and copy/paste everything)
Thanks again