There are two things to note before we begin.

• You will need to be a Guardian as the only way to drag spammers is by using Haikain's Grasp.
  
• You must not have the spammer blocked as you won't be able to send them invites if they're in your block list.

To freely pull the spammer around, you can either:
1) Duel them - although this might not work very well if your character is a higher level than them (as you'll just end up killing them)
2) Or my preferred method; inviting them to a party

Right click them and select either of these options to begin.

Once you have the spammer in duel mode or in your party, select them and drag them away using Haikain's Grasp.
The location you end up in should be somewhere away from all players. If you can't find anywhere like this, then try and drag them somewhere away from the main group of players. (i.e: In the Crossing, dragging a spammer to the far side of Elgren Farm would be the best as it is the furthest away from the main area and most deserted place in the Crossing.)

Once they are in a disclosed environment, you can either block them and leave them there or just leave.
Whichever option you choose won't matter as (if you've pulled them far enough away) you won't be able to see their messages wherever you are.

I know this takes some time and that the spammers will all reset at the end of the day, but if you're in, let's say Kellwoods, and there's a spammer sitting in in the middle of town yelping out rubbish, it doesn't take much effort to drag them away from the town so no one will hear them.
Sure, you could just block them. But then everyone else would have to spend time writing out "hejdiendisn" and "hdidowiqnsp" and "qosuendpzmq" into their block list. If the spammer is away from everyone, no one will even have to open their block list.

Please bear this in mind next time you see a spammer.