Can congress defund ObamaCare and over turn the supreme court law?

As a matter of law, Congress cannot strictly defund the ACA. As a functioning law, the ACA must be funded, though the degree to which it receives funds is given wide lattitude.

That is, Congress could pass a bill (and a President sign it) that removes all funding from all parts of the system required to make the ACA function. However, two things woudl still occur:

1. The law would remain in force, so which the government aparatus directly supporting it's operation would not be funded, the law would still have a significant effect, and all sorts of enforcement and operations would be carried out by the relevant departments.
2. A lawsuit would almost certainly result, and the Supreme Court would require at least a minimal amount of funding for the ACA. That is, the Supreme Court would force Congress and the President to at least make a basic effort to enforce and fund the law, as there is no question as to its validity (SCOUTS has priorly ruled that the law is Constitutional, and thus, must be enforced).

Second, there is no "Supreme Court Law" regarding the ACA. What the Supreme Court ruled was that the ACA itself was a legal use of Congressional Power.

Finally, the "Defund" attempts are nothing more than political stunts - that is, they are designed solely for media attention, and not any serious legislative work. No one would actually ever try to defund the system. Rather, if it became sufficiently unpopular (and a majority of Congress and the President agreed), then it would simply be REPEALED. As the different between defunding and repealing is virtually nill in terms of political requirements, defunding would never happen.