The first step in disabling a system hotkey that’s not listed in Keyboard Shortcuts System Settings is to run defaults read com.apple.symbolichotkeys.plist | less and find the relevant one.

Some filtering criteria that can be used are:

1. The third parameters value are modifiers (eg ⌥, Fn, etc). Modifier values can be found in https://gist.github.com/stephancasas/74c4621e2492fb875f0f42778d432973 and https://github.com/phracker/MacOSX-SDKs/blob/master/MacOSX10.6.sdk/System/Library/Frameworks/Carbon.framework/Versions/A/Frameworks/HIToolbox.framework/Versions/A/Headers/Events.h
2. The second parameters value are key codes. Key codes can be found in https://eastmanreference.com/complete-list-of-applescript-key-codes
3. The first parameters value are ASCII codes for the key. Since you’re dealing with function keys, that value will be 65535
4. If you’re trying to disable a hotkey, it’s safe to assume that enabled = 1

What does each part in com.apple.symbolichotkeys.plist mean? has more info on how to decode com.apple.symbolichotkeys.plist

Putting all those together:

1. Since you’re dealing with ⌥ and Fn, the third parameters value should be 524288 + 8388608 which is equal to 8912896
2. The second parameters value should be 107 for F14 and 113 for F15

        55 =         {
            enabled = 1;
            value =             {
                parameters =                 (
                    65535,
                    107,
                    8912896
                );
                type = standard;
            };
        };
        56 =         {
            enabled = 1;
            value =             {
                parameters =                 (
                    65535,
                    113,
                    8912896
                );
                type = standard;
            };
        };

To disable those, execute:

defaults write com.apple.symbolichotkeys.plist AppleSymbolicHotKeys -dict-add 55 "
  <dict>
    <key>enabled</key><false/>
    <key>value</key><dict>
      <key>type</key><string>standard</string>
      <key>parameters</key>
      <array>
        <integer>65535</integer>
        <integer>107</integer>
        <integer>8912896</integer>
      </array>
    </dict>
  </dict>
"
defaults write com.apple.symbolichotkeys.plist AppleSymbolicHotKeys -dict-add 56 "
  <dict>
    <key>enabled</key><false/>
    <key>value</key><dict>
      <key>type</key><string>standard</string>
      <key>parameters</key>
      <array>
        <integer>65535</integer>
        <integer>107</integer>
        <integer>8912896</integer>
      </array>
    </dict>
  </dict>
"

Then to reload the system hotkeys, execute /System/Library/PrivateFrameworks/SystemAdministration.framework/Resources/activateSettings -u