I'm using my dev org for testing proposes and It's taking my many time to delete objects, data, rules and so on.
It is possible to reset this account to original state?
You might look at using the Force.com Migration Tool to "undeploy" via destructiveChanges.xml, see more information here:
You don't need to - you can just sign up for a new Developer Edition account. You don't need to use the email address as the user name - for example, I have a user name of email@example.com with the email address of firstname.lastname@example.org. Don't forget that when you initially sign up with an email address that has to be your username - you can change it subsequently however.
Notwithstanding that, using a gmail account to manage multiple accounts is quite cool - gmail has a feature that you can put +anything after the name and it treats it as the same address - eg if my email address is email@example.com, I can send emails to firstname.lastname@example.org and they will go to the right address. Makes it easy to manage multiple logins.