Hi.
Do you know why I can’t remove Dynamic Servers??
It seems it’s blocked somehow… 
DHCP on WAN interface?
Untick “Use Peer DNS”.
BTW … change topic name as you ask for help “how to remove DNS”.
Actually the title is a more accurate description of the issue. The fact that he cannot remove the ISP DNS servers is a symptom of the fact that the OP doesnt know how DNS is designed on MT RoS and in fact he needs help setting up and understanding DNS.
Instead of being nit picky for zero value, I will at least post the documentation as a starting point.
https://help.mikrotik.com/docs/display/ROS/DNS
Granted this article fails to mention any connection to PEER DNS settings under IP DHCP client / pppoe client etc…
One has to reach the IP DHCP or pppoe client documentation to find this…
https://help.mikrotik.com/docs/display/ROS/DHCP#DHCP-DHCPClient
https://help.mikrotik.com/docs/display/ROS/PPPoE#PPPoE-PPPoEClient
( Sadly MT does not hyperlink between documents for such things )