As I understand it, I don’t think that solution would work satisfactorily. The bot only attempts to run the update script when there’s a new version out, which it detects by other means, so even if you made the script bail, I think it wouldn’t be run at the correct times.
The bot has three ways of detecting when a new version is available: watching Repology, watching GitHub releases, and periodically running every available update script in Nixpkgs. Regardless of how a package comes to the bot’s attention, if it has an update script, the script gets run and if it does nothing then the bot ignores the package.
So an update script configured to ignore certain releases ought to result in no PRs being created for that release.