How to prevent that user can enter only 150 characters

 <script type="text/javascript">
        function LimtCharacters(txtMsg, CharLength, indicator) {
            chars = txtMsg.value.length;
            document.getElementById(indicator).innerHTML = CharLength - chars;
            if (chars > CharLength) {
                txtMsg.value = txtMsg.value.substring(0, CharLength);
                alert("You can not enter more than 150 character");
            }
        }
    </script>

 <asp:TextBox ID="txtthink" Width="300px" MaxLength="150" TextMode="MultiLine" onkeyup="LimtCharacters(this,150,'lblcount');"
                                                runat="server"></asp:TextBox>
                                            <asp:RequiredFieldValidator ID="RequiredFieldValidator6" ControlToValidate="txtthink"
                                                Display="Dynamic" Font-Italic="true" ForeColor="Red" runat="server" ErrorMessage="Required!"></asp:RequiredFieldValidator><br />
                                            Number of Characters Left:
                                            <label id="lblcount" style="background-color: #E2EEF1; color: Red; font-weight: bold;">
                                                150</label>