Wally turns into a villain, fools the other Flashes and kills them (Barry, Zoom, etc...). There's no other speed force users on Earth (yes, I know Zoom doesn't use the speed force, stay on topic), who can stop him? I don't think heroes or anyone would destroy Earth to stop the Flash, so, let's not even go that route. Who stops an evil Wally?
Can he and if yes, then my answer to your question is no Earth destruction. Also, if you were Flash, I feel certain you'll go for Constanine first. Then, we are talking about a character that can be at multiple places at once.
Speed steal. What defense does Superman have against this? Also, didn't we just witness Superman struggling to keep up with two Flashes that were barely going all out?
Superman can power through molecular stasis, we've seen him do it. Speed steal in that fashion would be temporary at best. But moreover, stealing Supermans motion if you don't stop his motion completely could cause planetary destruction if he tumbles.
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TBH, I actually dont think Superman has an answer to speed stealing. But the combination of Superman and others( Batman, Constantine, Dr.Fate, Hal, Linear Men etc) should be enough.
Preparations+Haxed powers+Raw powers should be able to stop Wally
Also, Mxy and Bat-mite are like guardian angels or something to Superman and Batman right? So even if Superman and Batman cant handle Wally by themselves, the imps would stop Wally before he actually kills them