A few points...
The reason for that decision goes back to the late Cold War, before Germany's reunification. West Germany agreed mutually-beneficial gas imports from the USSR to promote 'detente' (peaceful coexistence) in Europe, but also co-operated with the US Reagan-government's arms-race approach in accepting US nuclear cruise missiles etc being based there. The USA and other NATO had combined carrot-and-stick approaches then, especially with Warsaw Pact Soviet-satellites like Poland... the USA sold low-price maize and wheat (that US farmers were subsidised to produce), despite that effectively being economic assistance to the Soviet bloc. Pre-Gorbachev, with Thatcher as PM the UK imported coal from Poland (and elsewhere) as it accelerated the shut-down of domestic coal-mining... imports that were really cheap, even if it was thanks to foreign miners living under a Communist-dictatorship, helped make British mines 'uneconomic'. When Solidarity was growing as an opposition and threat to the Communist government in Poland, a classic joke circulating was "Thatcher hates Unions, unless they're Polish"... they ultimately succeeded, but she'd been happy for British electricity bills to provide some money to the enemy.
Post-unification, that rationale remained for dealing with the Russian Federation after the USSR fragmented, with member-states like Ukraine becoming independent on top of East European countries becoming democracies highly-motivated to join the EU and/or NATO ASAP (like former right-wing dictatorships such as Spain had earlier). In addition, the political strength of Green/anti-nuclear opinion in Germany led to nuclear power being rejected as a kind of 'sustainable', non-fossil fuel generation... which helped make Russian gas becoming truly vital during its transition to carbon-neutral energy.
The result is that instantly shutting down Nord 1 would be an act of economic self-harm, exceeding that of Brexit (characterised as the first experiment in having a country with a globally-significant/relatively-large economy imposing sanctions on itself). Before this Russian invasion of/direct war against Ukraine the UK has imported a relatively small amount of Russian-produced gas on the basis of 'free-market' price/availability/convenience but nevertheless has its government only committing to phasing them out, not immediately banning them. It is entirely possible that that means an effective guarantee that the maximum amount allowed will occur if Russia cuts its price below the going global-market rate to retain foreign/hard-currency income... if the limits are applied in terms of GB£ value (say to minimise an addition to the current consumer price-hikes and/or raising the domestic price-cap before October) the volume imported could actually increase during the implementation of the policy.
In fact, shutting down Nord 1 from the Russian end is something Putin might use as a retaliation against the EU sanctions that Germany has agreed with every other EU member-state, making implementation and compliance with them binding under EU law and subject to oversight/scrutiny by the EU Parliament and Commision (unlike the UK's... subject only to the PM ensuring that they are enforced, as with anti-Covid rules).
Obviously, you're free to personally sanction Germany by switching from Eon, as an indirect sanction against Russia. But don't kid yourself that it will put an end to some of the money you pay for energy reaching Putin's regime to help fund his war of aggression.
To close a very much over-long and rather tedious post, I'll suggest that you (and anyone else considering that option) consider that Germany isn't just a NATO ally and undermining both that alliance and the EU is a big part of Putin's MO. But also directly and indirectly supporting and supplying Ukraine's population, government and military. For some reason that doesn't get as much coverage as the UK's in the media here, but I think you'll find it's also appreciated by Ukraine. Including by those of its population that have become refugees.
I can recognise that the impulse to do anything rather than nothing to oppose Putin's regime is entirely reasonable. I feel it myself. I just think that the rationale behind the action you've proposed doesn't make sense. If you want to switch for other reasons, fine. I'm no shill for Eon, I got assigned to them by Ofgem under the stupid, unnecessarily expensive system we have that is supposed to be all about consumer choice. Personally, I'd be happy if it was made into a customer-owned, not-for-profit operation... or the whole 'supplier' rigmarole was just eliminated through nationalisation... or something in between.