The phrase "a thing altogether" is correct and usable in written English.
It can be used to emphasize that something is completely or entirely a certain way, often in contrast to something else.
Example: "The project was a thing altogether different from what we had initially envisioned."
Alternatives: "a completely different matter" or "an entirely separate issue".
It may have a user override (Tube trains have passenger-operated emergency brakes), possibly mandated by the insurer, but you can just as easily see how an insurer would prohibit such a thing altogether.
For they can have no desire of offerings from the unwilling, unless they are animated by a spirit of contention, which is a thing altogether undivine".
Bear in mind that a cold or 'flu is a devil of a thing altogether, and may require a sizable measure of spirits to see it off.
But Samuel Barber's Piano Concerto, which was given its world première, with John Browning as soloist, was a different thing altogether — a modern concert work of considerable nobility, containing genuine inspiration as well as a great deal of complex writing of the "architectural" sort.
A set view of a harbor is a different thing altogether.
If you are carrying a musical, it's a different thing altogether.
Now, despite his outward resemblance to a human being, he is a different thing altogether.
If your close friends and trusted family members are telling you to change by stopping a bad habit, that is a different thing altogether.
There is no political cost to threatening a filibuster, but producing one is a different thing altogether, especially if it slows down or stops other important work.
Medicine may progress by analysing the brain, but - he insists - what philosophers need to explain is the mind, which is a different thing altogether, however much it depends on a properly functioning brain.
It is a movie about the art world, which is a different thing altogether.
