Make OS X Server a wireless bridge without DHCP
I'm assuming you have another device handing out the DHCP leases. If so, try modifying the DHCP settings on the Ethernet Interface directly, and then turn on Internet Sharing. To do this: 

System Preferences > Network, click the "Location" menu, and click "Edit Locations," name your new location, and click "Done". For Ethernet, click the "Configure IPv4 menu and select "Using BootP," click "Apply". In the left field, select "Wi-Fi," click the "Advanced" button, go to the "TCP/IP" tab and change "Configure IPv4 to "Using BootP" as well. Click "Apply," then go back to System Preferences and click "Sharing," and enable "Internet Sharing" in the left field. Set "Share your connection from:" to "Ethernet," and click the "Wi-Fi" box in the "To computers using:" field.