I got a B- in Math. How bad is that for Econ?
Is there anything I can do about this? 


I presume you mean "can do to change your grade"? No, grades are final (perhaps with an exception for mitigating factors). 


  Does the B- condemn me to eternal graduate school hell? 


I don't understand how a grade can condemn someone to "eternal graduate school hell," nor what "graduate school hell" is. If you are concern about graduate school admissions, then one grade will have little impact.