To be clear, CA has a write-in option, and as far as I know, candidates don't have to register to get it. It's just an "insert name here" thing. However (and again this is the first year that I noticed it) the option is only available for president and for local elections. Senate and Congress have no write-in option. For the senate, it's two Democrats. For Congress in my district, it's a Democrat and the incumbent Republican.
I'm almost tempted to write-in a candidate in the margins in the Senate section, but that would likely "mar" my ballot and they won't count it, so my only option seems to be the inability to vote for a Senator.