Problem with a bloqued field

Hello,
I've a table that enters by the product and that filters by this an entering, in which I add to lines and fields, like the one of client, with a series of conditions.
There is a boolean type field in which if it is clicked, the line is blocked. I want that when selecting a client compares by all the table if that client already has been introduced previously, and if the line is not blocked ,that does not allow me to insert it, but that if it is blocked leaves me.
I am with numerous problems, I have tried to put the field Customer.Nº as flowfield, but when forming part of a key me does not allow it; also I have tried in the C/Al with several instructions but nothing; I do not believe that it has special difficulty but I cannot with it; some aid please?
Thanks.