Legal Question in Real Estate Law in California

Absolutely yes. All they have to do is deed their share from themselves as joint tenant, to themselves as tenant in common. Now this may constitute a breach of some agreement among the parties, or be actionable on some other basis, but there is nothing to stop them from doing it.

Even their merely declaring they are no longer joint tenants with the other tenants or signing some document less than a deed [declining right of survivorship] can terminate the joint tenancy, although the law says it has no effect upon the remaining joint tenants as to their relationship with each other.

I agree. One joint tenant can unilaterally sever the joint tenancy.

There is a procedure for doing just that described in the Civil Code, section 683.2. This code section also contains some limitations on the manner and effect of severance in subsections 683.2(c)(1) and (2).