Thank you for your message.
Unfortunately, there's no feature that would allow automatically blocking outdated Viewers from connecting to the RU Server. However, you can try setting up the Centralized License Key Management
feature on the RU Server's side that allows to manage license keys centrally and assign license keys to connected Viewers. For example, it can be set up so that users would have to use a license key for version 7 in order to connect to the RU Server (and it wouldn't be possible if their Viewer is outdated since license key for version 7 is not compatible with Viewer of version 6). For more information on how to set this up, please see this blog post
Please let us know if you have more questions.