Q: i received message at my iMac saying i might have adware /spyware virus.

I have an apple iMac 2.5 ghz intel core is 4gb 1333  mhzddr3  with iOS X v.10 and we recently had a virus alert - Adware/Spyware virus appear on our screen and there was an alert to ring a Toll free number which we did and they wanted to remotely access our PC which we did not allow as we are very wary of having strangers access our PC.

How do we get rid of this alert and what do we need to do to protect our PC from viruses? We did not trust the Toll free number (they said they were Mac engineers) so how do we know if support team is trustworthy?