To impose a prohibition (aka: constraint, disallowed combination, etc.), for instance disallowing level 1 across attributes X1 and X2, in the in the Disallowed Combinations dialogue, use the following: X1 == 1 & X2 == 1 .
This same format can handle other (non-equal) prohibitions, as well as multiple attributes (for instance across three attributes as X1 == 1 & X2 == 1 & X3 == 1).
Separate prohibitions using a semi-colon.