No, there isn't. The upgrade nag is there because some user's email providers were blocking JRiver emails from being received, so they decided to do it through the app so potential interested users could find out about MC24's release. Even if they added an option to opt-out of the upgrade nags right now, it'd only be in MC24 and above and not older versions.
Disabling auto-update is still the best suggestion here as it's very, VERY unlikely there will be another MC23 release since all development for MC23 has ceased.