Your main mission isn’t to remove the warning you got, but to not do the same thing again, or something else that will get you another warning and perhaps account suspension.
Very rarely do we hear from sellers that Fiverr “took back” a warning. By all means, try to get it removed if you know and can prove that the warning was unjustified, but usually, if sellers get warnings, they did something that’s against the terms, like using pictures as gig images that they took from the internet, or just copied another seller’s gig description, etc.
I’m pretty sure that logo design is not a banned service on Fiverr. Find out what exactly it was you got the warning for, look closely at the mail you got, I can’t imagine it says “logo design is a banned service on Fiverr”, there must be something else, does it say something about copyright, for instance, or …? Check that and make sure to not do the same thing again, also read Fiverr’s terms of service thoroughly, so you won’t get a warning for something else that’s against the terms either.