How can I remove viruses?
Like above ...

If the file is persistent or un-deletable
 (usually the associated binary files under system > bin)
You need to use a Chattr first ..



busybox chattr -ia /system/bin/viralFile    



and then    



rm /system/bin/viralFile     



This is helpful for force removing virus or non-deletable files,  like most people on here that have used KingRoot or similar programs, it's a last option for hard to remove files !