To reproduce:
- Set the mask like this:
radMaskedEditBox1.Mask = "N0";
radMaskedEditBox1.MaskType = MaskType.Numeric;

Set the value to "12345" move the caret to the first position and press '.'
Workaround:
void radMaskedEditBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == '.')
    {
        e.Handled = true;
    }
}