Spim is unsolicited commercial instant messages or presence subscription requests, like spam, but in instant message form. While not by itself a compromise of the network, it's annoying in the least, can reduce resource availability and production, and can possibly lead to a compromise of the network. An example is users spimming each other by sending requests. Users can block each other to prevent spimming, but with federation, if a malicious actor establishes a coordinated spim attack, it can be difficult to overcome unless you disable federation from the partner.