Perform a find for all records with "CLT" and all records that do not have "CLT" will be omitted.
If on the other hand, you accidentally misstated that and want to omit all records that have "CLT", Specify "CLT" and make your find request an Omit request. There's a button to click for that in manual and stored finds.
If using scripted finds with set field steps such as those used here: Scripted Find Examples
use the omit records script step to turn the find request into an omit request.