Whatever you do, do NOT use any products meant for professional use, like effasol, or a stripper on your hair. There is a reason why they are supposed to be sold only to professionals. You will make the problem worse. Wash your hair with dish soap, and then condition well. Luckily, you used a semi permanent, which will fade mostly. As it is a dark color, it will take much longer to fade than a lighter color. Dark pigments hold on longer. Really, if I were you, I'd let the salon fix it, but I understand wanting to do it yourself.
Mrs. Rodgers
Tigeress A Salon