CydKnight
Explorer
Whatever man. I obviously don't know a tenth what you know if we go buy message board posts and signature dates but if it's within the rules as written, I just don't see the issue. I'm not trying to discourage players from doing anything but at the same time, spamming an ability through the entire game session seems like lazy gamesmanship to me. They are trying to make a spell or an ability act like passive perception when it doesn't seem it was meant to be used in that way. Still, if it's within the rules then have at it. As a DM I see it as my job to counter that when I feel it's necessary to keep the game challenging.But the game challenges are not primarily either "find a way to counter a PC's special ability which is one of their few limited resources" nor is it typically "hidden magic is a key to challenging PCs which is spoiled by detect magic".
Would you be looking for ways in the rules to make his offensive magic less harmful if he had chosen that option instead? It seems like you're kinda metagaming it a bit to act as a DM competitor. Why not roll with it and reward the player's use of detecting magic instead of choosing combat-oriented abilities?
If that is DM metagaming then guilty as charged but I know a few DMs that have been playing decades that do the same thing. Is it really a thing for a DM not to present challenges that test the known abilities of that games PCs? I am no "DM Competitor". I am actually for the PCs being successful and that actually makes it difficult at times to present them with challenging encounters without being so easy they are boring.
For the record this is simply what I would do in a worst-of-the-worst case scenarios after it has become a problem. I have yet to actually encounter this scenario personally and my answer may change depending on actual circumstances. In your games you should apply the rules as they work best for you.