Most BIOS's have a few settings that can make it more difficult for a physical attacker to gain entry to the system. These are:
Even if the BIOS is secure it can be reset back to the factory default by removing the BIOS battery from the mother board for a few seconds and inserting it back in. This can be prevented through good Physical.
If the machine has a TPM module and configured to use it, it is possible to detect these kinds of resets and prevent the system from booting into the OS afterwards, however, it won't protect against an attacker from booting their own OS. This can be mitigated by implementing some form of Encryption. If the hard disk can be encrypted based off a key stored in the TPM device, tampering would effectively permanently destroy access to all data on the hard drive (so you better have off-site backups).
Most new machines have a place for a padlock to be connected. An extremely determined physical attacker can either cut these off or if they are skilled in Lockpicking pick the lock. This shouldn't be necessary however in the machine is already in a physically secured location.
A large number of BIOS passwords can be bypassed using default passwords built into the system by the manufacturer. The system should be tested for these even if there isn't any way to disable them. A few of the common one are listed below:
It is a good idea to test and see if the BIOS has this kind of vulnerability.