Hmmmm. Not trying to be rude or anything, just want to provide some help here (hopefully i can be successful at this, apologies if I'm not), but...
That is limited thinking.
You've already convinced yourself that stopping the abuse on your system takes an insurmountable amount of work + expense ( 10 devs, 5 sysadmins over 2 months ). Sure, sure... it was "just a figure of speech" to convey a point. I get it. But... in the end it's really not.
This is you setting in stone that there is no other way to stop "the opportunity to abuse the application" as @rfctr put it.
Regardless of what IP ranges you ban, you'll still have this problem. You really need to "fix your app to not be trivially abuseable" as @mpalmer states.
THAT is the solution to your problem. Take our advise for what it's worth to you (if anything at all), but at least be open enough to the idea that you have not yet learned what you don't yet know.
Hope that wasn't offensive, but instead helpful in some way