You stop them once, they pop up elsewhere. I really don't think legal recourse helps here at all. These entities are usually coming out of jurisdictions which don't care about enforcing the relevant laws anyway.

Likewise if your open source project gets forked and redistributed with ads and other stuff onto the app store, it might mislead people in the short term but is likely not a viable long term strategy, especially if the ad ridden version is constantly out of date and missing features from the original.

On the other hand, if someone forks your project and does a better job of developing it than you, that's literally in the spirit of open source. If that entity then starts to distribute an ad ridden version, you are also welcome to merge all the useful changes they added to their fork and get on with life continuing to develop your ad free version which will likely quickly become more popular (even if all you did was re-distribute a more maintained version with all the ads removed, surely that's a good thing).

That is, unless you used the Apache 2 License and they licensed their contributed changes with something restrictive. Then you're screwed, just as is anyone who might want to pick up this project after/if FUTO stops existing.

I can see from the perspective of software which provides a networked service how developers of the software might feel "abused" because AWS or whatever starts selling their software as a service with no contribution back to the original project. But this is a mobile phone keyboard. No self respecting big company (except Microsoft, but they have no self respect so that rules them out anyway) would take an open source software project and re-publish a strictly worse version commercially. At least assuming you used an appropriately free license. To add to this, changes or improvements made by the commercial entity would be free for the picking for the original project.

Default good strong copyleft licenses are more appropriate for this kind of software, the Source First license as applied here appears to provide FUTO with very few actual benefits over something like the MPL2, GPL2+, etc in terms of discouraging corporations from re-packaging the code. It also does very little to prevent unscrupulous attempts to abuse the code to scam people into installing adware. The biggest impact it seems to have is simply in taking away user freedoms (which most users wouldn't have exercised, but which are still useful and important in the long term).