I'm no stranger to coding. It wouldn't be that difficult to add a feature that the dev can enable that marks the project as complete, or even on hold (which could extend the timeframe before a project can be flagged as abandoned) Once marked as complete, then the option for users to flag as abandoned can be disabled. It's really not that difficult of a concept.
Not to mention, there's already an "In development" marker on projects that are... Well, in development, it's not too much of a stretch to think that there's a way to change that "In development" marker to a "Completed/Finished" Marker. Which would obviously remove the ability to flag as abandoned.
I also seem to remember a search tag that allows you to filter search results for completed games only. ( I may be wrong here, because it's been a long time since i've used it)
These are just a few possible solutions that are already inherently built into the server's UI, or at the very least relatively easily modifiable.