new invalidChars[] = {"'", "\"", "#", "%"};
forward checkInvalidChars(text[]);
public checkInvalidChars(text[]) {
    new size = sizeof(invalidChars), found = false;
    for(new i = 0; i < size; i++) {
        if(strfind(text, invalidChars[i], true) != -1) {
            found = true;
            break;
        }
    }
    if(found)
        return 1;
    return 0;
}
public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(checkInvalidChars(inputtext)) {
        // Actiune, spre exemplu trimite un mesaj, afiseaza un mesaj pe ecran
        return 1;
    }
    return 1;
}