If an Athakan is played onto a battlefield and I have an Overzealous Fan there, can I bounce the Athakan back without triggering its attack ability?

No — you can't stop the attack trigger from triggering, but your bounce can make it resolve for nothing.

Playing Atakhan into a battlefield you don't control applies Contested, and since your Fan is sitting there, that stages a Combat. When combat opens, Atakhan is established as the Attacker and its "When I attack" trigger is put on the chain as part of that same combat-opening step (Rules 464.2.c–e) — before any player has a chance to play or activate anything, so there's no window to bounce it first. Your Fan's defend trigger activates at that same moment (it gains the Defender designation) and is placed on the chain after the attacker's, so it resolves first (LIFO) — you kill the Fan as the ability's cost and move Atakhan to base.

The trigger itself is already on the chain and still resolves, but by then "here" no longer refers to that battlefield, so the instruction can't be carried out and nothing gets killed — exactly the Fight or Flight example in Rule 359.3.f.2 (attacking unit moved to base in reaction, attack trigger mistargets). Note the Fan still dies as the cost either way, and you must choose Atakhan as the target when you finalize the ability.