"If [the US ban] does go ahead there is a very good chance that Russia will retaliate and stop supplying ... who's going to stop them"
You're saying that if/when the US bans Russian enriched uranium, Russia would respond by not supplying it – meaning it would basically comply with the ban...? I'm not sure what the affect / point of this would be, other than the Kremlin basically trying to do a "You can't fire me, I quit!" kinda thing, with its nuclear fuel exports..
I get that the US / West doesn't have nearly enough existing enrichment capacity to make up for the shortfall that would be created in that scenario. And developing such capacity domestically is a years-long proposition. Of course, none of this is a secret – Russian state media often highlights Europe and the US' dependency on Russia for nuclear fuel, along with calls for Rosatom's exports to these countries to be embargoed in retaliation to sanctions.
"They have many reasons to stop supplying USA and can do at a whim."
That being the case, the interesting question (imo anyway) is why hasn't the Kremlin already done it? I mean despite having the US over a barrel when it comes to keeping its reactor fleet fuelled (and "the lights on"), and despite the US/West sanctioning Russia to the hilt while arming Ukraine, the Kremlin continues to supply enriched uranium to US utilities (most of it with Centrus as the intermediary btw).
I don't know the answer (other than that it is not because of Russian benevolence or goodwill). Presumably it is at least partially because the current situation gives Russia leverage over the US; but if it halts the supply of enriched uranium, then it has basically used and loses that leverage. Secondly, I think Russia is desperate to get its hands on whatever USD (and EUR) it can.. There is a limit to what they can manufacture domestically, or import from North Korea or Iran, or buy with rubles, yuan or rupees from elsewhere, to mitigate the sanctions. Rosatom's exports to the West gives the Kremlin a rare source of legitimate USD/EUR inflows – and it seems it wants to preserve that, at least for now.
Also, had always kinda assumed that "if push came to shove" and Russia abruptly halted the supply, the US would have ways to make up for the shortfall even without enough domestic enrichment capacity, i.e. by downblending its stockpile of HEU. That assumption could be flawed, but anyway I guess this is a very long-winded way of saying I'm not convinced the lights go off in the US even if Russia suddenly embargoes nuclear fuel exports (but the wider uranium sector would get a massive tailwind out of it, that much is for sure ha)
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