ASP.Net如何在提交时删除/忽略特定元素的验证？

<asp:TextBox ID="txtUsername" runat="server" ValidationGroup="Group1"></asp:TextBox>
<asp:RequiredFieldValidator ID="rfvUsername" runat="server" ControlToValidate="txtUsername" ValidationGroup="Group1" ErrorMessage="用户名不能为空"></asp:RequiredFieldValidator>

<asp:Button ID="btnSubmit" runat="server" Text="提交" ValidationGroup="Group1" OnClick="btnSubmit_Click" />

protected void btnSubmit_Click(object sender, EventArgs e)
{
    // 删除/忽略特定元素的验证
    txtUsername.Enabled = false;

    // 其他提交逻辑...
}