We disabled automatic updates for High Sierra, so you can just install the version of Tropy you had previously installed and it should not receive updates anymore. My guess is it was version 1.14 but if that also fails you may have to go back to 1.13 instead. For example here is a direct link to the macOS Intel dmg file for 1.14.
Tropy will still keep checking for updates, but on High Sierra it will not receive them anymore. If you want to disable the update check entirely you can do so by opening the user data folder from the menu Help > Show user data folder, quit Tropy, and then edit the state.json file in the folder, setting the ‘updater’ property to ‘false’.