(SBT) How to disable default resolver and only use the company internal resolver?

Project level

According to the documentation we should be using externalResolvers:
https://www.scala-sbt.org/release/docs/Library-Dependencies.html#Overriding+default+resolvers

externalResolvers := Seq(
  "Local Repo" at "http://repo.alpinenow.com/artifactory/repo/",
  // some more internal Nexus repositories
)

Plugin level

You'll have to do it also in your project folder for plugins like in project/resolvers.sbt.

Global SBT level

And if you also want SBT it self to resolve from a specific repo, you'll need to do as described here: https://www.scala-sbt.org/1.x/docs/Proxy-Repositories.html

If you depart from the sbt-extras shell script as a replacement for the default launcher script, I guess you could easily modify that with setting up ~/.repositories and adding -Dsbt.override.build.repos=true. Then all you need to do is ensure your developers use that script.