Ross
We force it in our league. Anyone who doesn't use 2 tyre compounds and ends up in the points has to use a set of hards the following race. If they fail to do so and keep offending they get kicked. Simple and effe tive rule that works
We (Preston Racing) have something similar. Unless you are clearly running the default and you haven't set up at all, if you fail to use two compounds you must take a 'Stop and go' at the end of the first lap of the next GP. (a forced pitstop)