Under "Third Party", there is no "Remove" button. Instead, users must figure out that they need to go under "Installed" and find the same app in order to remove it. This isn't a bug, but it's awkward.
From my duplicate bug "The entry wont be listed under the Installed tab unless you close Software Center and open it up again, which means the only way to find it right after installation is in the Search tab."
Will be resolved with the new Software Center UX, which will see Third Party applications consolidated into their respective categories (like Chrome in Web Browsers) and utilize the same package view as native packages.
Consolidating with SC tasks on GitHub: https://github.com/solus-project/solus-sc/issues