Colpo di spugna literally translates to "sponge wipe" or "stroke of a sponge". It's an idiomatic expression meaning to wipe the slate clean, to make a clean sweep, or to forget about past mistakes and start anew.
In the context of the song, it suggests a desire to erase the awkward or stagnant moment the couple is experiencing and move on to something fresh. It's an interesting phrase that captures the feeling of wanting a fresh start in a relationship.
This song is about that awful, quiet moment after everything has changed in a relationship. Ligabue describes a scene where just a minute ago, things were different: "Un minuto fa / Eravamo qua / Era diverso" (A minute ago / We were here / It was different). Now, the air is thick with things unsaid. The singer and his partner are stuck in a "Pomeriggio stupido" (stupid afternoon), feeling exposed and disconnected.
He perfectly describes the awkwardness of the situation with small, telling details. There's a fly that's been around so long it must have "fatto il nido" (made a nest), and his partner's face is only lit up by her phone screen. The song shows how a single minute can feel like a lifetime and change everything. It's about that feeling of numbness when you know something is over, and you just want to get through the pain as fast as possible, like pulling a tooth.