Many "professionals" say that disruptive behavior should be ignored and positive behavior should be rewarded. (ie- When student is beign disruptive the teacher will say look at how nicely elizabeth is sitting quietly and doing her work. Great job., etc.) I have not always found that this works.
Is this child always disruptive or are there times when he is cooperative? If he does something good or behaves praise him tell him I am very proud of you that was very good that you were able to sit still through the entire story, etc.
If you are really at your wits end and the assistant is not able to help you control the situation, to me it would seem that you need to tell the mother he is not ready for a class setting.
As the teacher you need to make the recommendation. Be sure that you make it clear that you are not demanding her to take him out, but you are suggesting that at this time she might want to reconsider his enrollment. You also have the responsibility of the other children and his disruptive behavior is going to impair the other children's experience in Sunday School. Good luck.
“Infatuation is when you think he's as sexy as Robert Redford, as smart as Henry Kissinger, as noble as Ralph Nader, as funny as Woody Allen, and as athletic as Jimmy Conners. Love is when you realize that he's as sexy as Woody Allen, as smart as Jimmy Connors, as funny as Ralph Nader, as athletic as Henry Kissinger and nothing like Robert Redford - but you'll take him anyway.” -Judith Viorst