How can I remove a file which doesn 't allows me to uninstall other app?

Hi, I owe an older MacBook Air 13-inch, Mid 2011, 1.GHz Intel Core i5 with memory only 4GB. I m running on OS X El Capitan version 10.11.6, and everything is running fine. I just found out I m not able to trash an app.

I have a problem with removing iBooks author.app. I tried to remove it by using iCare app, which I use to erase unwanted apps. The application can not remove it, it says I have to find ClamXav folder im my laptop than to find a file called uk.co.markallan.clamxav.plist and I shell remove it by confirming it with my system password. So I found it and I just could move it to trash so I did. Thy system didn't ask me for password. I googled the link "uk.co.markallan.clamxav.plist" on web and the reccomendation was to delete it in system .. and also download from clamxav a remover program so I did.

But nothing really happened. I can still not remove the iBooks author app for the same reason. If I want to crosscheck if there is no more files clamxav, I can not find them.

Can you please help me somebody? What is the problem?

Thank you very much in advance

M.