Just check "Web-based" and uncheck "Ask me every time".

No, I would like to know if this can be enforced server-side.
Reason: there are multiple users that will be using the web console & the Remote Control feature,
and I don't want them to be able to select the legacy option...

For that, maybe someone from Soti in the know can tell you if such option exists.  Otherwise, you might need to make a feature request. 

It is also possible that legacy remote control based on plug-in will be phased out after a version of html5 remote control released can perform most, if not all,  functions of the former.  By that time, there won't be any choice.

Hello Steven - Can you please contact SOTI Support and create a case for Feature Request. Thank you!