You can follow the below 3 step for allow access or blocked site for specific IP in MikroTik Router
1– configure Web proxy on your Mikrotik. go to IP >>> firewall >>> NAT >>> clik on (+Sign) to add a NAT rule. in Chain: choose dstnat in Protocol choose 6 (tcp) in Dst.Port Write 80 then go to Action Tab in Action: Choose Redirect and in To Ports Write 8080
now you have done first step.
2– go to IP >>> Webproxy >>> clik on Webproxy Settings click on enable check box to enable Webproxy settings.
3– now block www.google.com go to IP >>> Webproxy >>>Access (From right panel) click on (+ sign).
from Dst.Host write www.google.com and in Action choose deny,then google will not be access. then in Redirect to write www.itjagat.com when the try to open www.google.com they automaticly would be redirected to www.itjagat.com.com
4. Allow any IP Address you want.Go to IP >> Webproxy >>> in Scr.Address write the IP and in Action: choose Allow
by the way you can filter by the content ( wave. mp3. zip .exe and much more… content you want )
with all due my respect ENJOY