I am barred from entering my new iMac, due to iCloud!

I bought a new computer 2nd hand. The guy who sold it to me did some very nasty thing to my email account since he lost temper with me after a discussion. He set in motion his iCloud account on his old compouter. The guy boast w. pride that he is an expert in hacking&cracking and he knows very well what he sis doing...

Ther is now information what so ever on this, no safeguardiung rules that you should ALWAY format the internal drive when you have bought a 2nd hand mac... Is there a workaround this? It seems to me that tharae are no way you can do any procedures during start up....?

I am tired of looking at a machine I cannot use...

macmanner