The plug-in started as simply a way to block disposable email addresses, originally all it did was query undisposable.net as to whether the address was valid or not. Due to constant problems with undisposable (which actually seems to be working at the moment), I added an internal list of disposable email addresses.
The site-admin does have full control over which domains to block. If you want to remove the internal list entirely, you can simply select them all and then delete - the list supports multi-select, so this is fairly painless. It will still check undisposable though. Once the list is clear you can add your own domains as you wish.
I will at some point add an option to skip the undisposable check.
Funnily enough, since I wrote and installed the plugin, I haven't had many people trying to sign up with disposable email addresses anymore. The ones that do seem to then continue to try and access restricted parts of the site and other dubious behaviour... I have had to ban a few IP addresses because of this.
When you say you had a bounced spam signup... do you mean the plugin worked as it should (i.e. didn't let mail.ru sign up once you had added it), or it actually let it through?