Ban on both.
While I see the thought process, this really isnt true at all. Even with preparation, mgar destroys stall; I ran an mbounce dark type, 3 ghosts, a shed shell, and chansey, with pivot moves on all but one and magic coat on chansey in case I got cought untransformed by mgar, such as versus an opposing chansey. I still had three of my 4 losses entirely because of it. Obviously I could have prepared a little more, like by running bounce instead of unaware on registeel, but if you have to run both an item and ability on every mon on the entire team thats seriously a problem, especially since mgar isnt the only pokemon in the metagame. In general, theres almost nothing a stall team can do against Knock Off+Mgar, especially with hazards, without being able to do absolutely nothing else.Adding on to the above, team preview is a huge aspect when considering the potential of trapping as although Illusion does somewhat skew the presence of Stag Gar, aggressive pivoting/double switching can actually pressure the Gar from actually netting any KOs, especially considering if hazards are up and the accumualtion of chip damage from weak attacks it usually can only switch into. Of course Shadow Tag Mega Gengar is going to have a field day if one chooses to keep spamming recovery moves or status moves in general that do not affect it, but it would be more wise to "play ahead", especially if Stag Gar can disrupt your team strucure into a disadvantageous state.
I am not undermining the importance of sustaining defensive pivots with recovery moves however, and will admit that a good Stag Gar player can often force their opponent into 50/50 positions, but I still feel thats more of an issue in the teambuilding stage and not one that should only be looked at in battle if you catch my drift.
Here are a pair of replays demonstrating what im talking about:
http://replay.pokemonshowdown.com/gen7balancedhackmonssuspecttest-596140611
http://replay.pokemonshowdown.com/gen7balancedhackmonssuspecttest-596147319
Yes, if I hadnt made that mistake with yveltal I might have done better, but it wouldnt have mattered; it could just have come in while I used rapid spin to remove rocks, or anchor shot at chansey, or recovered, and then I would have been in the same bad situation. The idea that you should have to build your entire team to not autolose to a single stallbreaker that can still take an enormus toll on you anyway is just absurd: this pokemon isnt remotely balanced for the tier.
As far as magnet pull goes, its similarly uncompetative. Klangs set is just one example, and it means that running chansey gives the opponent a 20% chance to win every turn registeel is in, and there will be nothing you can do about it- not to mention that registeel is a very bulky pokemon, and in almost every game it will get dozens of free turn on the feild, and that a phazing move on registeel is viable anyway. Magnet Pull also does the same for any steel type, making it so you have to have shed shell or not be able to use it all game - thats an s rank, 2 a ranks, and 3 b ranks you cant even use should they have this one ability that you have no idea whether or not they are running until you run straight into it. Obviously Primal Groudon is the best user, but its far from the only one, and the ability is independantly broken.