Well, Hitler did a lot more than just invade Poland. He also unilaterally reoccupied the Rhineland (which was demilitarized) and stomped all over the Treaty of Versailles. He then annexed Austria (the circumstances surrounding the Anschluss were not exactly looked upon favorably by the Entente). And then, after a fair bit of saber-rattling, he oh-so-graciously condescended to avoid war with Czechoslovakia as long as Czechoslovakia surrendered large tracts of land (which also happened to be integral to its defensibility). And then Hitler invaded Czechoslovakia anyway.

So the German invasion of Poland in 1939 wasn't an isolated event. It was sort of the point at which even the most war-weary of states realized that Hitler wasn't going to stop until someone stopped him and that you couldn't negotiate with someone like him.

Stalin's invasion of Poland (and Lithuania, Estonia, and Latvia) was looked upon poorly, but Stalin, having declared war later, was seen as more of an opportunist than an aggressor (remember, the secret annexes of the Pact where the Soviets said they'd invade weren't well-known at that point). The Entente definitely weren't happy about it but they also didn't want to push Stalin into a closer alliance with Hitler by declaring war on him. Especially since there was no conceivable way they could actually attack the Soviet Union without diverting stupendous amounts of resources.

When Stalin invaded Finland a few months later, however, things changed. The Entente was openly considering an expedition to Finland to support the Finns in their war with the Soviets. And if that just so happened to mean that the Entente would also occupy part of Norway and northern Sweden (specifically the region where much of Germany's raw iron ore originated from), well, clearly that was just a coincidence, right? Well, no. Norway and Sweden weren't okay with that turn of events and they refused.

There was also talk of bombing Soviet oil fields in modern Azerbaijan in an attempt to disrupt the Soviet military's combat capabilities. Nothing came of this, though.

Ultimately, though, any attempts by the Entente to punish the Soviet Union for its invasion of just about every country it bordered went down in flames after France's disastrous defeat in 1940. With France effectively knocked out of the war (barring a few Free French forces and some partisans), the British had their hands full trying to stave off the Germans, nevermind starting a war with the Soviets that they were not in a position to fight (let alone win).

A Gallup survey in 1939 revealed that more than 80% of the British public supported an alliance with the Soviet Union in case of war against the Germans. An alliance between the West and the Soviets was in fact not unthinkable as modern day readers make it out to be.

The problem was that the Soviets had decided to essentially ally with Germany after the Munich Agreement; since the British and French had essentially abandoned their Czech ally without even consulting them. Why should the Soviets ally with the British/French if they may end up doing the exact same thing to the Soviets for instance?

That the Soviets and Nazis were essentially already allied in 1939 was simply not recognized by the West. It was only after Finland that it was clear they were working together, which resulted in the expulsion of the Soviet Union from the League of Nations.