Send a letter via snail mail that clearly states that you disagree with their EULA but were unfortunately only made aware of the EULA after your purchase. Include a printed, revised EULA with your proposals for changes. Be reasonable in the changes you propose.
If they reply to you that they disagree with the changes, they can prohibit you from using the software or service, but then you're also entitled to a full refund, of course.
If they forget to reply, their EULA will not be enforceable.
Even if they haven't replied, the original contract is still sitting there clearly stating that a certain set of assumptions are in place if you use the software. I don't see any way to argue that a non-reply implies consent to new terms or abandonment of any terms.
Are... are you... this is the internet and are you... you are serious? Can I believe this? I want to believe this.
You know, the things that nobody reads but generally sign away all your rights.