This question already has an answer here:
I was recently deemed "undesirable" in South Africa for overstaying my visa. I want to visit my South African fiance in Botswana but all of the affordable flights stop at OR Tambo. Will the gate agents for my connecting flight before SA know I am not allowed in the country and prevent me from boarding? I don't want to get stopped in London/Zurich and not have Christmas with my fiance.