by using bridge interface..
To block DHCP IP and to avoid the costumers from using another public IP, follow these steps :
A. After adding the bridge and its ports, go to Filter tap, and add a filter..
B. 1- Chain select Forward ..
2- Click on MAC Protocol..
3- Apply, to open the IP field..
C. 4- Write the public IP in Src. Address and Dst. Address..
5- DON’T forget to check the box beside the IP field.. (!)
6- Go to Action Tap..
D. 7- from Action select drop..
OK..
NOTE
If the costumer have more than one Public IP, simply add a Bridge Filter Rule, by repeating the steps from A to D.. and::
E. Click on Ingress Priority and enter (1) in the field, for each rule you have added..
Note:: enter the same value (1) in every rule to make sure that all the rules have the same priority..