Block the hostname prod.cloud.rockstargames.com (e.g. using the hosts file, the firewall, or router).
This unfortunately makes all online components unavailable (iFruit, Returning Player Bonus that everyone has been given access to, etc.), and I recall having launching issues at times, but if you can find a way to automate it for use with Online, it may be very convenient.
Going offline works, of course, but Rockstar's DRM either outright refuses to launch the game, or only launches it within a time window after the game was last launched with connection to Rockstar. The aforementioned issues apply here as well.
@BlueStarBRS That may be detected as tampering by the game.
Looking for a better solution myself, as well.
Edit: look at this, deleting "GTAV\mods\update\update.rpf\x64\data\cdimages\scaleform_frontend.rpf\gtav_online.gfx" gets rid of it entirely! Source. A bit inconvenient it's yet another step to do each time while modding a new update.rpf, but I think it's worth it.