How to uninstall Net Nanny which has blocked access to finder

I have a huge problem. I installed Net Nanny on my grandchild's new Mac Air laptop, Yosemite OS. On installation I see an almost blank screen - a box for login and a box for password. I typed in the login and password. Nothing happened. I am sure it is correct. I gave up and cancelled my account and was given (am in the process of being given) a refund. I now cannot uninstall it. I was first told to go to applications and uninstall it...etc. I cannot get into Applications...or anything else in the computer. I was told next to login with my email and given an admin password. Nothing happened. Each morning (I am in Australia) I wake to find a similar instruction..this time "the admin password is ....... " So I type Admin and the password. Still nothing. I even was going to buy it again ...hoping that it would solve the problem. I am glad I didn't.

It seems the people at Net Nanny don't even read what I say....I cannot get into the finder. I cannot even see my dmg installation file on a usb. I think I may have to resort to reinstalling the system software, which I really don't want to do. He hasn't had the computer for long and not much will be lost ...but there will be changes he will notice. If only I had done the back up before the upgrades.

Has anyone else had this problem? Does anyone have any ideas? I have searched and searched for answers... Is it perhaps incompatible with Yosemite?

Thank you

Lori