Government enforces it. I don't know how exactly it is in the USA, but in my country there is "land registry", which is maintained and updated by legal courts. Within the registry, every land/house/flat ownership is noted and this registry is also the final instance whenever there are legal battles regarding ownership.
Whenever you take a loan for your house, a bank will want to add a note to the registry that it has first claim to your property in case of not paying, premature death, and so on. So one cannot just give his house to his relatives, as the court will block the transfer when it will see in the registry that the bank has primary claim to it. To do something like that, one must make a deal with the bank that new owner will either pay up the loan or take it on his name.
I believe there's a limit to how much you can give away to your heirs while still alive. Probably if you give more, you have to pay extra tax. And I can imagine that if you gave away a lot just before your death and ended up with a lot of debt, debt collectors may still be able to get the gifts back, but I'm not entirely sure about that; I'm no expert on this.
It seems like in the USA, gifts to a spouse are exempted from the gift tax[1]. I don't know about USA, but here (Finland) any unusually large gifts given just prior to death are considered a part of inheritance.
[1]http://en.wikipedia.org/wiki/Gift_tax_in_the_United_States#N...
why couldn't you (as the inheritee?) "throw away" all assets prior to dying, then let your benifactors refuse the debts (which is all that's left)? Who actually enforces that you can't dispose of your assets (in a way that benefits your son/daughter/spouse), then refuse the debt?