rule 8-2-2
 

team A throw a legal foward pass that is intercepted by team B. Before the change of possesion, Team A blocks below the waist. Team B returns the interception for a score. will team B decline this penalty to keep the score or will it be enforced on the try or kick off?

8-2-2 talks about plays where there is no change of possesion so no option to penalize on the kick off. B has to decline A's hold if they want the ball and the TD.

Don't have my books with me, but I believe that if team A had fouled AFTER the change of possession, B would be allowed to keep the score and the penalty options.

Should be noted that this may be a state interpretation here in NC, and not necessarily applicable to your state. If I had my books and notes with me, I could tell you for sure.

Canadian Ruling
 

CANADIAN RULING:

An illegal block below the waist is not a major foul (it's a minor foul), and applying a minor penalty before a change of possession gives the ball back to the original offensive team, but (in this case) 10 yards back.

B will decline the penalty altogether to keep the TD. The R should not even consult with the B captain.

REPLY: That's correct for Fed rules too independent of any NC interpretations.