There is a simple answer to this. Stop the joint primaries and go to a system most states use where a Democrat primary selects a Democrat candidate, and a Republican primary selects a Republican candidate - or however many parties wish to be on the general election ballot.
Of course Democrats control California so they created this joint primary system so they could keep any Republicans off the general ballot guaranteeing on e of them will win. This system is in direct contrast with a NJ Supreme Court decision requiring NJ to replace Bob Torricelli on on the ballot for Senate when he had to drop out 60 days before the election due to a fraud charges indictment in conflict with NJ law that said no changes to the ballot that soon before an election. The Supreme Court ignored the law and the Democrats put Frank Lautenberg on the ballot and he won. The NJ Supreme Court said NJ voters had a right to a choice which superseded NJ law!
Apparently, US voters only have a right to have a choice of Democrats on the ballot. This is the democracy Democrats were afraid Trump would destroy.