Hello!
Maybe there is a better way of doing this but what I did to get rid of RTs is:
I added a text filter for RT (in caps and including a space after RT), and check words and case. This is to make sure that I don't leave out a tweet like: "SHORT". Then I will select OR option. This will give me all the RTs. However, I save this to a separate file and double check, just in case and if you want to be really thorough.
Then, to see the messages without the RTs, instead of the OR option, check the NOT. And this filter will output those messages without "RT ".
I hope this helps!
Best wishes,
Almudena