Shoutbox

Plus! never provided a function to remove the text; rather, it could replace the text using the registry:

HKCU\Software\Patchou\MsgPlus2\your@email.com\Preferences
   

ChangeSecurityWarning (DWORD) Default: 0
SecurityWarningTxt (String) Default: (local specific)
If ChangeSecurityWarning is set to true (1), the security warning displayed at the beginning of chats is modified to match the text specified in SecurityWarningTxt

---

It is MSN Messenger itself that removes the text: the registry key "HKEY_CURRENT_USER\Software\Microsoft\MSNMessenger\PerPassportSettings\(your passport id) -> WarningMsgCount" matters. See CookieRevised's reply to Get rid of the warning text at the start of chats.