[Kyance]

You can just check if the player is logged in or not when they type a command/something in main chat.

For the dialog:

pawn Код:

ShowPlayerDialogEx(playerid, dialogid, style, const caption[], const info[], const button1[], const button2[])
{
    ShowPlayerDialog(playerid, dialogid, style, caption[], info[], button1[], button2[]);
    DialogOpen[playerid] = 1;
}
(!) Reset "DialogOpen" at the "OnDialogResponse" callback.
And do it on the very top of that callback. ( Dialog[playerid] = 0; )

Should work