How can I disable private browsing/incognito mode in browser X?

OK I'm presuming you;re using Windows...

In Internet Explorer:

Click Start, Run. (or in the search bar for Vista or W7) Type GPEDIT.MSC and hit ENTER Go to:

User Configuration / Administrative Templates / Windows Components / Internet Explorer / Privacy

Turn off InPrivate Browsing:

In Firefox:

Open up your hard drive (c://)(or in the search bar for Vista or W7) through the 'start; run' route. go into program files, go to mozilla firefox, then components.

NOW, make sure you are searching for all files under 'files of type'.

Then, you will be able to see "nsPrivateBrowsingService.js"

Right click and DELETE!!!

That won't remove the controls, but they won't work any more. If you want to hide those Menu items: Add this code to userChrome.css below the @namespace line. See http://kb.mozillazine.org/Editing_configuration#How_to_edit_configuration_files

#sanitizeSeparator, #privateBrowsingItem, #sanitizeItem {display:none!important;}
#privateBrowsingAutoStart {display:none!important;}

Source: http://support.mozilla.com/tiki-view_forum_thread.php?locale=en-US&comments_parentId=378652&forumId=1

In Chrome:

Creating a DWord reigstry key called IncognitoModeAvailability, with a Value of 1 for Disabled in the registry folder HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome seems to disable incognito browsing. Thanks to Imomata for that one.

I'm unable to help you on the Safari one as I have never really used it for any length of time

You can disable Incognito Mode in Chrome with a simple registry tweak. In regedit.exe, create the key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome, with a single dword value, called IncognitoEnabled and with value 0. This has been deprecated, the correct dword to use is now IncognitoModeAvailability, with Value 1 for Disabled.

When you next start Chrome the New Incognito Window option will be greyed out.

Or you can create .reg file by opening up notepad, pasting the below in and saving it as eg incognito.reg. Once saved just double click the reg file to inport it.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Google\Chrome]
"IncognitoModeAvailability"=dword:00000001

On Chrome for Mac: (Edit: this should work for Linux, too)

Create a plist file in /Library/Preferences/ called com.google.Chrome.plist with the following:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
   <key>IncognitoModeAvailability</key>
   <integer>1</integer>
</dict>
</plist>

(If the plist file already exists you need to add the dict entry (the thing that says IncognitoModeAvailability followed by the value 1.)