Deleting a connection is as simple as creating one. Think carefully before doing so, though. All cards created or connected via this connection will 'lose' their connection once it's deleted.
1. First step is opening the settings sidebar by clicking on the gear icon in the upper right corner of the pipe dashboard;
2. Scroll down until you find the connections option;
3. Click on the three dots next to the name of the connection you wish to delete;
4. Click on "delete connection", confirm and that's it, your connection is gone.
Once the connection is deleted you'll no longer be able to search/create connected cards.