Then, your spacer is far too thick! As is specifiet in the ISIS specification, there shall be a press fit connection between the male and the female tapered spline of about 3 to 6 mm, normally around 5 mm. In practice this means, when you attach your crank arms by hand, there shall be around 3 to 6 mm space between crank shoulder and spacer (if not: take a thinner spacer!). Then you tighten it till there is no gap between spacer and crank left to reach the proper amount of press fit and then up to about 40 Nm which will tense up the screw enough so that it can not loosen itself.
The pedaling torque is transmitted by the splined interface. The taper ensures that this splined joint has enough pre-load and no play. The spacer ensures, that the crank will not move further on the spline with time (caused by widening of the crank) and allow the bolt to become loose.
If you need finer increments in spacer width, check out Shim Rings DIN 988 22x30 available in 0.1, 0.2, 0.3 and 0.5 mm. (see here) But they’re only needed to adjust the position of the disk brake rotor if you use and EDB. Cause as desribed, preload can be between 3 and 6 mm, a pretty wide range. If your gap (untightened!) without any spacer is less than 6mm, you don’t even need one.