Translation:I am not interested in football anymore.
Good for me as a German to memorize. "Nicht mehr" has the same double meaning as "больше не" - "not more (of sth)" and "not anymore (unlike before)". Though I guess "not more (of sth)" would be "не больше"?!
To my ears there is no difference indeed. The speaker says that his limit of interest is now reached. English seems to make an additional difference between amount and timeline:
I do not have (even) more interest (than I still have anyway).
The time for me being interested in football is over.
"I couldn't care less" expresses complete indifference. You don't care and probably never cared. The fly on the window is more interesting than [it]. In Russian you can say for it "мне совершенно/абсолютно всё равно", "меня это абсолютно не волнует", "меня это совершенно не интересует", "мне это безразлично".
"Я больше не интересуюсь футболом" means that you used to like football before but not anymore. Now you are interested in some other things. Больше = anymore, any longer.