How to delete unknown contacts from whatsapp?

[shree09]

I add my friend's gmail account on my sony xperia c4 mobile with gmail app, after 1-2 days I found many contacts are uploaded in my whatsapp contact list and they all are start with letter A and then a two or three digit numer like e.g. A 5869, A 605 . like these and I can see their whatsapp profile also but cant see their real name.
when I check my phone contact book there were no such contacts uploaded so I though this may be cause of I add my friends gmail account on my phone so I remove his acoount, delete my app data for whatsapp, and again restart my whats app acoount I got added in groups in which I previously added thats ok but when I see in whats app contacts those contacts are still there.
I clear app data for phone contact book, remove my frineds google account, clear whatsapp data again but still problem is there yet.
I can't delete those contact from my phonebook, when I trying to delete it notification popup says "Can not delete following number : A 5068" like this.
please help me .
is my phone hacked by someone or affected badly by virus?
I'm using Nortan antivirus in my phone.
android system is Marshmellow 6.0