1st way is to try log into your Samsung account via findmymobile.samsung.com , once logged in it will hopefully show your registered Samsung devices... new and old. from here you can remove keypad locks or even wipe the device.
2nd way is to hard reset by pressing a certain button combination, like Pablouno says you can youtube the method. its normally having the S7 switched off then pressing down the volume up button, home button and power button all at the same time for around a 10 seconds... it will come up with a menu, scroll down to wipe device... but after you do this and the phone reboots it may trigger something called FRP (factory reset protection) thats where you will have to input your registered Gmail email account and Gmail password to prove you're the original owner... after you pass that then the phone is ready to use.
