Do you have an adminstrator account and password?
If so, update Flash Player.
1. System Preferences > Flash Player > Advanced > Delete All
Press the "Delete All" button
Install Adobe Flash Player.
http://get.adobe.com/flashplayer/
Follow the prompts. Click Safari in the menubar and select “Quit Safari”.
Restart computer. Relaunch Safari.
Otherwise you have to reset your computer to the factory settings.
Giving away / Selling your Mac / Resetting the computer to the factory condition
http://support.apple.com/kb/HT5189
http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/