Try this, in Admin -> Settings -> User
Users will still be able to upload their own avatars though. There isn't a setting to disable that, but for the time being you can set "Maximum User Image File Size" to 0. Created an issue for that specifically:
https://github.com/NodeBB/NodeBB/issues/3013
psychobunny created this issue in NodeBB/NodeBB closed Ability to disable custom avatar uploads #3013