As Dr.bloodSpinat2000 said, "Tools > Manage > Storefronts to disable" the Enable automatic Imports on Startup to stop games from Stem, Epic, etc.. The other games will not show up again if properly deleted from LB. Make sure that the "allow deleting ROMS when deleting games is checked" in Tools>options at the top right, otherwise it will reimport the games you just removed or manually delete them in the games directory and scan for removed games. If you don't want to permanently delete other ROMs by accident make sure you uncheck the "allow deleting ROMS when deleting games".
