The judge can prevent a jury from considering whether a defendant's actions in a homicide (or less destructive use of deadly force) were justified (or excusable, if you prefer) as an action of self defense, and if he or she does so, the jury is prohibited from finding that the homicide was justified by that reason. The question that the jury must then answer is whether they find beyond reasonable doubt that the defendant committed the act.
I would say if the judge prevents the jury from doing their job, then their only proper response is to acquit, no matter the facts of the case.
Never forget that juries are one of the few protections a citizen has against a tyrannical or abusive state. Those abuses can come from unconstitutional law, over-aggressive law enforcement, or from courtroom judges who suppress evidence or give unwarranted or illegal orders to juries.
As a juror, you have more power than the Supreme Court. So you can safely ignore any blather from the bench. Do not let them narrow the path. Judge the case in totality, starting with - is the law Constitutional? Is the law fair? Was it applied fairly in this case? Is the law applied differently to different classes of people? Was the accused subjected to excessive violence or other unwarranted abuses?
It's up to you to see that justice is done, and you get to decide what is just. If you don't think "sting" operations should be legal, acquit. If you don't think "no-knock warrants" should be legal, acquit. If you don't think DUI checkpoints should be legal, acquit. It's pretty simple.
There is a proud American tradition of jury nullification. It was used to abolish fugitive slave laws, and alcohol prohibition. It's been used in civil rights cases and modern drug cases. 3%-4% of jury trials involve nullification. Please have the courage to vote your conscience.