Our toilet flushes so vigorously that water splashes out of the top of it. Is there a way I can "tune" the flush so that it doesn't flush so hard?
For adjusting the water level in the fill tank, all of the toilets I've come across are adjustable, there is no need to put something in the tank to offset the volume.
There will be a float of some sort that is attached to a valve. To have less water fill the tank, you want to adjust the float to sit lower in the tank, thus less water is needed to push the valve closed.
For the two fill mechanisms I'm familiar with:
For the float arm style there is either an adjustment screw or you can bend the arm.
For the newer float on the pipe style, see here. Again, simply the attachment between the float and the top should have an adjustment, move the float lower in the tank so that a lower amount of water pushes the valve closed.
In most toilets (those that aren't pressure assist), there's a float that will stop water from the tank at a given level. There's usually some adjustment to change the level that the toilet will fill to.
Placing something into the tank is used to reduce the amount of water that's used per flush, but as there's still the same depth of water per flush, it won't change the initial pressure of the flush, which is what's going to give you the "vigor" behind the flush.
Pressure assist toilets are another beast -- I've never had one, so I have no idea how to tune them. They fill a sealed container, so rather than being exposed to the air, they have the full line pressure to use in the flush. I would hope that there'd be something in either the manual for the toilet, or the manufacturer's website on how to make adjustments.
If you take the lid off tank, and there's no water visible, then it's likely a pressure assist. If there's no tank ... well, it's likely drawing from line pressure entirely, as they do with commercial toilets. (actually -- I saw a lot of tankless toilets when I was in Europe ... with the flush mechanism in the wall, so it's possible that there's a tank in the wall ... but I have no idea how you would adjust and/or service those).