I was going to remove an AD Forest from AD Connect and took for granted that this would be an easy operation to do from within the AD Connect wizard.

I realized that it is just possible to add AD Forests. I than thought that miisclient would be the place to do this. There is two solution to this that I know off.

1. Uninstall and reinstall AC Connect.

The other that I prefer is described here.

Disable the AD Sync Scheduler by running this PowerShell command: