"You have no empirical tests to establish or confute the idea of intelligent design."
Then, by definition, it cannot be science. Untestable claims cannot be examined by science.
"You can declare it to be "unscientific," but not on a scientific basis."
On a definitional basis. On an epistemological basis. On a practical basis. What good is a claim you can't test?
Yeah. What good is the claim that the bioshpere is essentially the result of an evolutionary history from molecules to organisisms to arrangements of organisms to self-awareness as experienced by the only biological entities known to practice science and write it down?