Spammers are trolls who deliberately spam the same sentence over and over (usually their name) and annoy people. Some people copy and paste to spam, some just type in short nonsense words (like "asd") quickly. Spammers sometimes don't give up for a while and spam until everyone in the room leaves out of anger. Sometimes, they hide in a safespot, like under a box and underground. Quite a few fancy-lettered and other notable players have spamming as a second job.
The spam does not have to be the same sentence. If the sentence spammed isn't copy-pasted, then the spammer might type random nonsense.
They might even use hacks to help them spam faster or more lag-inducing. If spammers are using hacks to spam, whether it's an immortality hack or a spam hack, it would be impossible to kick out a spammer. That is, until the spammer gets bored.