Hi Aleksey.
First of all, thanks to answer my question.

I had tried to use the ERROR() function insted of MESSAGEBOX and RETURN .F.
It works fine if your code call explicit the TABLEUPDATE function. But the problem that I mentioned is when you move to other record (for example, if the user click in other line in the grid) then VFP will fire the TABLEUPDATE. And in this situation, if you use ERROR(), VFP will jump the RETURN .F. line and will update the record anyway (it shouldn´t happen, because I raise a error using ERROR()).
I try to do something in the Error event of CursorAdpter, but with no solution.
I didn´t find any other way to stop the tableupdate that was fired by VFP instead of run a RETURN .F. in BeforeUpdate event. And it give the "bad" message that I mentioned.
Maybe I am doing something wrong, or I didn´t undertand well your idea.

What do you think?

Thanks.
Ricardo.