[Konstantinos]

Keep in mind that returning 0 prevents the player from spawning in OnPlayerRequestSpawn.

pawn Код:

// global variable:
new
    bool: Logged_In[ MAX_PLAYERS char ]
;

// OnPlayerConnect:
Logged_In{ playerid } = false;

// When a player logins:
Logged_In{ playerid } = true;

// OnPlayerRequestSpawn
if( !Logged_In{ playerid} )
{
    SendClientMessage(playerid, -1, "Please login first!");
    return 0;
}