[Threshold]

Instead of relying on the client's response, you should try and keep everything server-sided if possible.

For example:

PHP Code:

#define DIALOG_TEST 1

#define MAX_LIST_ITEMS 25        // The maximum amount of list items
#define MAX_LIST_ITEM_TEXT 60    // The maximum length a list item text can be

new
    ListItem[MAX_PLAYERS][MAX_LIST_ITEMS][MAX_LIST_ITEM_TEXT],
    Player_ListItem[MAX_PLAYERS] = {0, ...}
;

#define AddListItem(%0,%1) format(ListItem[(%0)][Player_ListItem[(%0)]++], sizeof(ListItem[][]), (%1))
#define ClearListItems(%0); for(new litem = 0; litem != sizeof(ListItem[]); litem++) ListItem[(%0)][litem][0] = EOS; Player_ListItem[(%0)] = 0;

public OnPlayerSpawn(playerid)
{
    AddListItem(playerid, "1. Briefcase");
    AddListItem(playerid, "2. Torn Letter");
    AddListItem(playerid, "3. Cyanide Pills");
    AddListItem(playerid, "4. Toolbox");
    ShowListDialog(playerid, DIALOG_TEST, .caption = "Items List");
    return 1;
}

ShowListDialog(playerid, dialogid, caption[] = "Dialog")
{
    new dialog_str[MAX_LIST_ITEMS * MAX_LIST_ITEM_TEXT];
    for(new i = 0; i < sizeof(ListItem[]); i++)
    {
        if(ListItem[playerid][i][0] == EOS) break;
        format(dialog_str, sizeof(dialog_str), "%s\n%s", dialog_str, ListItem[playerid][i]);
    }
    return ShowPlayerDialog(playerid, dialogid, DIALOG_STYLE_LIST, caption, dialog_str, "Select", "Exit");
}

public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(response)
    {
        switch(dialogid)
        {
            case DIALOG_TEST:
            {
                printf("%s | %s", inputtext, ListItem[playerid][listitem]);
                if(strcmp(inputtext, ListItem[playerid][listitem], true) != 0)
                {
                    // inputtext does not match the list item text they selected
                    //Kick(playerid);
                    return 0;
                }
                ClearListItems(playerid); // Reset the player's list items text
                return 1;
            }
        }
    }
    return 0;
} 


And I repeat, that is purely an example. You should make something that fits your own needs.