Gmail id block

I don't think you can delete a gmail account, but if you simply don't want to use it anymore, set it with a stupidly long complex password and stop using it.

If you want to delete the account you can go here:
Canceling your account
You account ID will become available to other people once you do this.

If you want to lock the id so that no one else can can get it then I would go into account settings and change your secondary email account to be the same as your gmail account. (assuming they will let you do that) Then change your password to a bunch of random key strokes. Now any time you (or someone else) tries to retrieve the password it will be sent to the gmail account. If you can't set the secondary email account to the same as your gmail account then open an account on hotmail/yahoo, set that secondary account to the gmail account and set the password in the same fashion.