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.