Thank you pandeyism but this didn't answer the issue. I found by myself the solution. The issue is that the server side doesn't know that the validation has been disabled on client side so you need to add to override the validation using the following code.
Public Overrides Sub Validate(ByVal grp As String)
Dim enableValidators As Boolean = CheckBox1.Checked
valName.Enabled = enableValidators
MyBase.Validate()
End Sub
And then in the submit button add the following the code.
Page.Validate("Group1")
If Me.IsValid Then