Edited

//oben im Script
#define DIALOG_SUPMOBIL

new Supmobil [MAX_PLAYER_NAME];

//OnPlayerDisconnect
//unter OnPlayerDisconnect
if(supmobil[playerid]) DestroyVehicle(supmobil[playerid]),supmobil[playerid]=0;

//bei den anderen ocmd's

ocmd:supmobil(playerid)
{
    if(supmobil[playerid])
    {
        DestroyVehicle(supmobil[playerid]);
        supmobil[playerid]=0;
        SendClientMessage(playerid,ROT,"Dein Supmobil wurde erfolgreich gelöscht");
        return 1;
    }
    if(GetPlayerState(playerid) != PLAYER_STATE_ONFOOT) return SendClientMessage(playerid,ROT,"Du musst zu Fuß unterwegs sein!");
    return ShowPlayerDialog(playerid,DIALOG_SUPMOBIL,DIALOG_STYLE_LIST,"Supmobil","Sultan\nCheetah\nSandking","Wählen","Abbruch");
}
//OnDialogRespons
if(dialogid==DIALOG_SUPMOBIL)
{
    if(!response)return 1;
    new Float:p[4];
    GetPlayerPos(playerid,p[0],p[1],p[2]),GetPlayerFacingAngle(playerid,p[3]);
    switch(listitem) {
        case 0: supmobil[playerid] = CreateVehicle(650,p[0],p[1],p[2],p[3],100,100,-1);
        case 1: supmobil[playerid] = CreateVehicle(415,p[0],p[1],p[2],p[3],100,100,-1);
        case 2: supmobil[playerid] = CreateVehicle(495,p[0],p[1],p[2],p[3],100,100,-1);
    }
    SendClientMessage(playerid,ROT,"Du hast dir ein Supmobil gespawnt!");
    return PutPlayerInVehicle(playerid,supmobil[playerid],0);
}