Cant change next time -checkbox

I have a checkbox.First i checked,then i want to change the check. But it's possible now

view

                                            <div class="control-group">
                                                    <label class="control-label">Language Known</label>
                                                    <div class="controls">
                                                        <label class="checkbox">
                                                            <input type="checkbox" value="1" name="lang_known"/> Hindi
                                                        </label>
                                                        <label class="checkbox">
                                                            <input type="checkbox" name="lang_known" value="2" /> French
                                                        </label>
                                                    </div>
                                                </div>

I want to disable this..No one will be allowed to change that checkbox value once selected.

For this what i do in my code..