To display the overlays currently enabled by the cape manager, type:

cat /sys/devices/bone_capemgr.*/slots

On a default Beaglebone Black, it should show:

 0: 54:PF---
 1: 55:PF---
 2: 56:PF---
 3: 57:PF---
 4: ff:P-O-L Bone-LT-eMMC-2G,00A0,Texas Instrument,BB-BONE-EMMC-2G
 5: ff:P-O-L Bone-Black-HDMI,00A0,Texas Instrument,BB-BONELT-HDMI

The format of output is:

 Slot Number:
 Slot I2C Client Address:
 [P]robed Flag
 Probe [F]ailed Flag
 [O]verride Flag
 [l]oading Flag
 [L]oaded Flag

Capes can be forcibly enabled or disabled by passing the following parameters to the kernel during boot:

capemgr.enable_partno=BB-CAPENAME
capemgr.disable_partno=BB-CAPENAME

Kernel parameters can be specified in the uEnv.txt file.

For example, if you want to disable the HDMI interface to free up I/O, mount the FAT partition found on the onboard eMMC where the uEnv.txt file is located:

mount /dev/mmcblk0p1  /media/card

Add add the following to the uEnv.txt file:

optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN

BB-BONELT-HDMI is the on-board HDMI interface. BB-BONELT-HDMIN is the same interface but with no audio. If only the BB-BONELT-HDMI interface overlay is disabled, the BB-BONELT-HDMIN is loaded instead hence the requirement to disable both.

Then umount and reboot the bone.

Now, when the list of overlays are displayed, the Bone-Black-HDMI & Bone-Black-HDMIN interfaces are shown as not loaded (L):

 0: 54:PF---
 1: 55:PF---
 2: 56:PF---
 3: 57:PF---
 4: ff:P-O-L Bone-LT-eMMC-2G,00A0,Texas Instrument,BB-BONE-EMMC-2G
 5: ff:P-O-- Bone-Black-HDMI,00A0,Texas Instrument,BB-BONELT-HDMI
 6: ff:P-O-- Bone-Black-HDMIN,00A0,Texas Instrument,BB-BONELT-HDMIN