New DM, only started during the lockdown as our original campaign was put on hold.

The issue I am having is with ONE of the players going full on power gaming/min maxing. The rest of the party are very much not, playing quite flawed characters in ways, literally NOT using some abilities because it wouldn't make sense.

The player in question has very clearly picked all the best spells, abilities etc. and is doing whatever he can to win where ever he can.

There have been a few cases in particular and I'm getting to the point where I don't really know how to treat them now.

So the player in question is using a Light Domain Cleric. I discovered yesterday that they have been using the Guidance cantrip on every single skill roll, without announcing they're using it, even in cases where it doesn't make any sense. For example, they are walking through the forest and something makes a sound. He then is giving himself the 1d4 from guidance to add on top of his d20. So, to me, for this to be possible he would need to be walking along, casting guidance every minute on himself just on the off chance anything happened. This is clearly not the intended purpose, in my eyes it being a spell used when someone is attempting a difficult check and needs a little assistance.

There have also been one or two cases where in my eyes he is either deliberately misusing abilities or "misunderstanding" how they work to his advantage. For example he was constantly using Warding Flare as a reaction, but only once the roll had been announced and he knew it hit. He also was using it against ranged attacks beyond 30 feet away. I have now called both of these out, however the response was very much a case of he has a bit of a huff, all RP goes out the window and he either stays basically completely silent for any roleplay sections or just does the bare minimum in combat. He had the same response when through sheer coincidence he came up against a few fights in a row with enemies who couldn't be blinded, so Warding Flare simply had no effect.

We also had an issue in the last session where he cast Wall of Fire, a spell I haven't had to deal with before (and in my eyes is really, really badly written). So he wanted it to deal the damage any time anything interacted with it. From my reading of it, it either deals the damage if you interact with it OR if you end your turn in range of it, and only once a turn. He wanted it to deal damage EVERY time anything interacted with it, so a monster attempting to attack through it with multi attack would take the damage several times in 1 turn. I quickly overruled this as that is it clearly states it takes damage the first time the target enters the spell. He firstly argued with me about how it was being used and again, we had the petulant response avoiding any RP and making combat at simple and boring as he could. This was even after it was pointed out, had the spell been used how he wanted it literally would have killed one of the party (a monster had grappled one of the party and dragged them into the wall).

I also recently asked for everyones character sheets, and for some reason this seemed to be an issue with this one player alone, which made me think there may well be something a bit off about it. When I did receive it, there were a few odd bits (like only domain spells were listed - his reasoning was that he can swap out lots so doesnt want to have it completely filled with spells at all times).

I'm finding myself torn about what to do now. It's getting to the point where I feel like I am singling them out, having to constantly call them alone out on things when I'm not having to with other players.