I don't see any options in the BIOS to disable video output. You might consider using the NVRAM jumper on the motherboard to reset the server back to factory default settings, though. Page 3 of the User Manual shows the location of the jumper. You would need to cover the common pin and the uncovered outside pin, then power the server on. You can then power it off and move the jumper back. If the problem is a setting related issue, that should fix it. You may also try a power drain by unplugging the server and holding the power button down for 30 seconds, then powering back up.
If you've already performed both the power drain and the NVRAM clear and gotten no behavioral change, I would recommend both taking the server to the minimum to POST (described in the manual I linked to previously) and using an alternate monitor and cable. Using different video equipment can rule out the external issues, while taking the server down to the minimum to POST should reduce the variables to the bare minimum. If the issue stops, you can add components back in until video halts again. If the issue continues, you would know that the issue is with something that you can't continue without (like a motherboard).