As far as I know you can force server resource packs to be required by the options
You can probably use this to just force the vanilla resourcepack
I don't want to force the vanilla resource pack as I know some server mates use chunk border resource packs and other utility packs. Playing with the chunk border pack when not doing anything technical is a pain. I don't want them to stop using resource packs altogether but I do want to limit the resource packs to make it fairer for everyone playing.