Do you "have" to?

Not voting is a form of expression as valid as any other.

You really don't have to accept a very bad candidate and by doing so you signal future candidates the availability of "free" votes.

Now, I admit that voting NULL is a stronger message, but is it really an option?

No. They are the people saying I need to choose between them. Supporting either is not in my interest, they will not earn my vote by being slightly less abhorrent.