[Taqwus]

For what it's worth, you don't need trade restrictions or tariffs in the real-world sense (the in-game "tariffs" are really tribute), because there is no inter-empire trade by default.

Unless you enter into a trade treaty or any higher-level one (Trade, Trade & Research, Military Alliance, Partnership), no trade takes place. With any of those treaties, trade generates bonus resources -- initially 1% of the other guy's resources, which are free (not taken from his production), increasing by 1% per turn up to 20%.

If you want to completely embargo somebody else, you could attempt to disrupt their trade (via intelligence -- which is broken until the patch comes out), or you could try to persuade its partners to break their treaties.

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-- The thing that goes bump in the night