How to make bitcoin-qt respect -prune? I'm using Bitcoin Core 0.18.0-rc2 (64-bit) on Qt 5.9.7.

How can I make it respect -prune=550?

This is the minimum prune number, and should be approximately 550 MB.

Yet, my client is downloading tens of gigabytes worth of data, with no end in sight.

In "Options" it is stated:

"Options set in this dialog are overridden by the command line or in the configuration file: -prune=550"

Hence, it seems like the program is able to recognize the command line argument. It simply doesn't respect it as far as downloading and pruning the blockchain goes.

What can I do differently to fix this problem?
prune allows you to set the size of blockchain data stored, not downloaded. Therefore prune does not affect bandwidth usage.