If you had a colleague who was consistently writing complete shit you would raise it with your manager. This situation isn't all that different - the only complicating factor they're not on your team.

If it's only happened a few times you might first try setting some ground rules for contributions. Really common for innersource repos to have a CONTRIBUTING.md file or similar. Add a checkbox to your PR template that the dev has to check to indicate they've read it, then wait and see.