i deal with a lot of nerds that are all interconnected with one another and nobody can agree on a common subnet for their houses when we all decide to exchange routes internally. ;)
Crucially, we set aside the common 10.1 and 10.10 ranges as nonroutable, so any devices that default into those have to be dealt with before they can live on the wider network.
Newcomers get handed a block and have to renumber, or NAT into it, or whatever, but they cannot emit bogons. At a small scale (first-name basis), this works.