If you are getting 100s of unauthorized transactions in your Authorize.net account, it means your Authorize.net transaction key got stolen.
There are reports of this kind of hack is going on out there.
You need to do this:
1) Scan your computer for a malware. Looks like a hacker got access to your email and/or your store control panel.
The best maleware scan we found is this one:
Download a free version, do a full scan and let it clean your computer.
2) Change your email account passwords
3) Change all administrator passwords in the store ( in Users -> User search )
4) Login to your Authorize.net account, change login password there and generate new API key
5) Copy that new key and past it in Settings -> Payment methods -> AuthorizeNet - SIM -> Configure
6) We can setup and additional level of authentication for your control panel in the store but it will not help if you have a keystroke logger spyware on your computer . Hacker will get that password right away so you need to do a first step of cleaning up your computer. contact us after you did that.