What do you mean how did they disabled?
So the PS3 when ir shipped has an option to install Linux or other OSes (practically just Linux, as there were a handful of Linux distributions that supported the PS3 hardware including, IIRC, Yellow Dog Linux, which was originally a fork of old Red Hat Linux, ported to run on PowerPC CPUs that Macs had, which the IBM POWER Cell CPU in the PS3 was similar to. Then for some stupid reason, I think two or three years after launch, they published a firmware patch which disabled the Other OS feature by modifying the bootloader* so it would only run signed code, which you
had to install if you wanted to continue to use Multiplayer or play any titles released after the update. Also all new PS3s sold after the firmware patch was released had it installed. So at that point, people with a PS3 made before the patched firmware could either choose between installing the patch and getting the full gaming functionality, or continuing to run Linux and loosing the ability to play new games, upgrades to old games, or multiplayer.
*On all computers there is firmware that initially loads the operating system. On older PCs, this was the BIOS, which would load the Master Boot Record, which was 512 bytes, which for some OSes was enough to put the bootstrap code in, like DOS, whereas with others, the code in the MBR loads a “stage 2 bootloader” which loads the OS. Old versions of Windows, like 3.0, 3.11, Windows 95, Windows 98 and the much despised Windows ME, kept DOS around as a second stage bootloader. Windows NT, the advanced multiuser OS initially released in the early 90s, upgraded soon after its release, then as NT 4.0 with the Win95 GUI in 1996, and as Windows 2000 (NT 5.0) with the Windows 98 GUI in 2000, and as Windows XP (NT 5.1) in 2001 with the Luna desktop theme, and Windows Vista (unpopular) and 7 (very popular) with the much loved Aero Glass theme in 2006, then the hated Windows 8 and 8.5 or whatever it was called in 2011/2012 I think, with the unpleasant Metro GUI, and now Windows 10, which is tolerated, which has continual uploads, have their own bootloader, which was pretty much the same limited bootloader until Windows Vista, which added the ability to boot other OSes, and Windows 8, which added encryption.
Most other hardware, including Intel Macs and various servers, and consoles like the PSe, and modern PCs, support firmware that directly loads the entire OS without the use of a 512 byte Master Boot Record on the hard disk and first and second stage bootloaders separate from the initial firmware (BIOS on older PCs); the firmware does everything. Most non-Intel CPUs made recently, such as SPARC and POWER servers, use OpenBoot firmware, which is quite powerful, whereas Intel and AMD CPUs use a popular firmware called UEFI, which is extensible. There is also a fully open source firmware compatible with a limited range of systems, GNU CoreBoot, which is awesome when it is available. But needless to say gaming consoles never want you to have that kind of power, as a rule, and the PS3 was a rare exception in that its firmware allowed it to load another OS, until Sony released a patch disabling this.