You can do this with a Snow Leopard DVD. Apple sells this for $20 but I think they only continue to sell it for Mac OS X upgrades since you need to have 10.6 installed at the bare minimum to upgrade the OS through the App Store. 10.6 is EOL and many applications no longer support it, so there is no practical reason to use it unless you have a specialized use case for it.
In order to boot from this DVD, put it in the drive and press Option and select the DVD or just press C and the system will boot from DVD. Both of these will work, so it's a matter of which one works. Expect to sit for ~15-20 minutes or more just to boot up the installation DVD.
After it loads, go under Utilities and find the Password Reset utility. This should clear the password on the system.
Generally speaking, if this is a HDP (Hard Disk Password), many companies just replace the drive and do not try to reset the locked hard drive. Many drives encrypt this data, which makes HDP removal impractical. You may be able to crack it and get it off of the drive, but this operation is very risky and could damage the drive beyond repair.
Being I changed this answer to not read as if I wrote it like I don't care about the quality, I need to update this. The old methods (Change the RAM and reset the PRAM or using install media) do not work on 2011+ machines. It is coded into the EFI ROM chip, so you need to take the system to Apple to have the machine reset. The problem is you need a proof of purchase, which may be an issue. The TPM in these machines was never used, so you don't need to worry about the TPM interfering.