IIRC, some trade does still get through, though at a dramatically reduced rate, and regardless of the trade ratio. The resources committed, however, are still 'traded' (i.e. lost) at 100%. If you are running a healthy economy* and are maxing out w/ coal and/or steel at 99,999, offering excess to the world market can be a good way to pick up some extra rubber/oil. Remember, whenever a state of war exists, you stop trading (except at this reduced amount). If you anticipate a short war, don't delete your trades on the world market, as you'll lose their priority (equal trade ratios are prioritized per date initiated). Once the general war begins w/ UK & France, delete these so as not to needlessly lose resources.
* The trick to a 'healthy economy' is to work out your total IC expenditures (consumer goods + supplies + production + research) to be equal or less to what your resources allow for after conversions (and allowing for daily military oil usage), NOT maxing your industrial capacity (except for the short term when you have sufficient reserves). The basic resource consumption rate for 1 IC is 2 coal, 1 steel and .5 rubber; excess coal is converted to oil, which is then converted to rubber, per technology levels and automatically (you can't control it). While these conversions may fluctuate between any given day, if you set an IC limit & stick with it, they do average out. Again, that self-imposed limit does not have to be as high as the stated industrial capacity ...
If this is too much information or you already know this, I apologize, but I do hope it helps ... Personally, I love this economic micromanagement aspect of the game!
P.S. This info is the same for HoI 1.06 and Core 1.91