My problem is that grew xprox. 50 cabbages and 50 cauliflowers
and I have lost them ALL to Cabbage Maggot.

Ouch. But identifying the pest is one of the hardest parts, so you
are ahead of the game.

A google search for cabbage maggot shows
http://pmo.umext.maine.edu/factsht/Cbgmag.htm (and a lot of other fact
sheets, probably similar). Advice there includes removing remnants of
the dead plants this fall, possibly tilling the soil this fall
(although there can be downsides to that too), and a collar around the
stem of any host plants you try to grow next year.

If you plant any brassicas next year (cabbage, cauliflower, broccoli,
brussel sprout, etc), and in fact any of the hosts for this insect
(radishes, rutabagas, turnips are listed above), I wouldn't put them
in the same soil as the ones which got munched. If you can bear to
avoid all those crops for a year and grow something else, I might try
that.

I would also plant a larger variety of plants. This helps with pests
(rotating from bed to bed each year can throw them off, and if nothing
else you'll probably find *something* you grow is less susceptible).
Also, if all of your cabbage and cauliflower plants *had* produced,
all at the same time, you probably would have gotten mighty sick of
eating those things. Think in terms of maybe half a dozen to a dozen
crops (making it *too* large, especially at first, means you have more
to learn about each one).