What can be done to curb illegal Mexican immigration

I keep hearing that building a fence (i.e. Berlin Wall) is not
feasible. I also hear that it's impossible to gaurd this huge border
between the USA and Mexico. In that case, what are the most
cost-effective ways of preventing illegal migrants from Mexico entering
the USA?

I have a solution on what to do once they get here: Put them to hard,
value-added work, and then once the costs associated with this is
recovered, then send them back.

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