My workaround was to go to Resource > Manage added data sources > Edit

I added a new dimension called 'Exclusions'

I used a Case statement in this to set the field value to 'Exclude' for items I wished to exclude, and 'Include' for items I wanted to include.

I then set the Exclusions field in the drop down list, and set the default value to 'Include'

Instructions for the case statement: https://support.google.com/looker-studio/answer/7020724?hl=en#zippy=%2Cin-this-article