I’ve mailed Solo campaigns with SLM in the past, is there a way to suppress those recipients from receiving my Poplar campaign?

Addresses that have been mailed to in the past can be uploaded as an Audience for suppression. Next, head to the Suppressions tab under your campaign to select the Solo list for suppression.

To suppress an Audience list across all campaigns, upload to your Do Not Mail list, which is automatically suppressed across all campaigns.