About all you can do is call SpamCop and try to get your domain removed. It's highly doubtful that they will clear the other group, but they *might* clear yours.
Getting listed for spamming sucks. I have an opt-in only, non-adult marketing company and we got listed once. It was actually a mistake, which was cool, but it still took a lot of calls and 'splaining to get us removed. It's no fucking fun...
There may be other ways to get unlisted, but I only know of that one.. YMMV.
Good luck!
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