Anything done to stop unwanted junk email from reaching you — advertisements, scams, mass mail you never asked for. The usual methods are a blocklist (a list of senders whose mail is refused outright), a filter that reads the message and guesses from its wording whether it is junk, and rules that watch how a sender behaves. You will meet it written three ways — spamblocking, spam-blocking, and spam blocking — and all three are read the same; the hyphen is the safest choice in writing because the closed form still looks new to many readers. It works mostly as a modifier in front of another noun: spamblocking software, a spamblocking service, spamblocking rules. It is technical, workplace vocabulary rather than something you would say over dinner. Note that it names the activity or the technique, not the tool — the tool is a spam blocker or a spam filter. The word 'spam' itself is worth knowing separately: it began as a brand of tinned meat, then a comedy sketch where the word was repeated until nothing else could be heard, and that is the image — the same thing over and over, drowning out everything real.