The EU would have to rely on German, Austrian, French and Italian cops/soldiers to actually enforce any sort of physical blockade against Switzerland.

I am not certain if all the respective governments would simply obey such an instruction if they didn't have any beef with the Swiss themselves.

Which now means that the EU has to block not just Switzerland, but (say) Austria, which means compliance from four other countries (CZ, SK, SI, HU), which is even less probable etc. etc.

Ultimately, the EU is really good at processing papers, but doesn't have the power necessary to initiate physical punishments of this sort unless the relevant member states agree.