Freeing up space · Caches and clutter
Can I stop one app from filling my whole disk?
Short answer: Move that app's data to your fast drive — the Apps window or What's Using Space can do it with a link left behind — or let its cache be cleared automatically. Storage Studio warns you if it starts growing fast again.
The practical answer is to move that app's data off your built-in drive. The Apps window can relocate or offload an app together with its data to your fast drive, leaving a link so the app carries on as normal, and What's Using Space can move a large data folder on its own.
If what's growing is a cache, letting it be cleaned automatically is the easier fix — caches are meant to be thrown away.
Storage Studio keeps watching afterwards, so if the app starts growing quickly again you'll hear about it early, with the app named when it can be identified. See the Apps FAQ and which app is filling my disk.