Thanks krileon for yr suggestion but after trying this the plans are invisible and there is no option at all.
I will keep playing around.

ok after several weeks and a few more hours of trying every configuration imaginable I am convinced this can not be done with CBsubs. If anyone can suggest otherwise I would love to know. So just to make sure I have been clear previously this is what I am trying to achieve:

1.Create 2 plans that exist subscription opyions (but not from the registration process).
2.Disable the alternative plan from selection after 1 plan has been chosen.

That's it, sounds simple but I am positive it can not be done.
I hope I'm wrong:)