Just build the fence, and call it a day.

Too difficult to enforce.

Sorry, but this is just about the most stupid idea yet. We'd spend more money trying to track down this money than we'd collect. And these people are desperate. The families are desperate. If the money is being sent south, it means they aren't planning to come up, so what you'd do is force the whole family up. Most importantly, you're punishing the wrong people.

Are we talking about funds sent by both lawful and unlawful immigrants? This would probably be problematic in its implementation and probably impractical in the long-run, and would do little to halt the rate of illegal immigration into this nation. It is worth nothing that the vast majority of individuals crossing the border illegally are here seeking employment to earn income to support their families back home--- this policy would most likely not stop illegal immigration but result in them attempting (with likely success) to bring their families in with them. This seems counter-productive to the overall goal we are striving for.