Which gets us nowhere, since it is still begging the question: How?
The obvious answer everyone comes up with is something soft. Think inner-tubes on the DIY cheap end, Seismic Shelf or whatever on the spendy end. The problem with all of those being they still need to be tuned or balanced if you prefer.
Less obvious is use something sufficiently massive and solid. Go massive enough and at some point the thing simply will not be capable of moving at any but the lowest subsonic frequencies.
Its even possible, if you want to go to all the bother, to calculate based on dimensions and mass exactly what those frequencies will be. Or you could just build one, jump up and down enough to get it rocking, and make a pretty good estimate. Mine is way down in the low single digits.
There's another one the isolationists never seem to answer, and that is the question of: Which matters more- vibrations that are external, ie a part of the environment? Or internally generated?
The answer by the way is: Both. In many if not most, or maybe even all situations the turntable itself is the greatest source of vibration. Given the wackiness of some contributors this might be a good time to say for the record I don't mean during earthquakes. But most other times, just sitting there playing a record, even in your rickety room, the turntable itself is your main concern in terms of vibration control.
Which means, of course, that regardless of what you use or how perfectly it isolates you still have to deal with the question of the very first material the component rests on. Because whatever that is, is going to determine more than anything else the nature of the vibrations reflected right back up into the table.
That's why vibration control is not just words to say, but rules to live by. Isolation is only one aspect of vibration control. I seriously doubt it is even the most important.
