Super sorry to hear this happened to you! There are definitely people who are just in it to ruin things for others - "ignore" is a great handy feature for these such times, and as others have said, if you see players breaking the rules, be sure to report them in-game!Community Manager for Jagex
had it a few times just continue what you where doing they'll eventually move on.
i was in slayer dungeon killing abby demons some maxed out guy came in said this is my spot started killing away so i continued he says stuff like i was here first (again i think not as i been there a good 25 minz before he showed up) i stayed he started with the go away noob i'll kill you , and ofc i stayed continued ....and it went on and on i'll hunt you down scrub and so on and so on for about 15 minz until he switched lol.
just keep on doing what your doing eventually they'll get bored.
I just want to add on to this, don't act like you own the spot you're at either - it's not your personal location. As I mentioned in my first comment back when all this game's servers were almost always full (dependant on timezone synchronisation) you either stood your ground and didn't give in thus you silently agreed to shared with the person and/or people or you left.
You had to share because it didn't matter where you went everything was being used, you'd get lucky and maybe have less than 5 minutes until the next person showed up; but I'm telling you now there was hardly ever a single spot that wasn't in constant use.
For an example: I don't care if you're where you are, if I'm there to do the same thing and there's more than enough for another person I'm training there too. No one owns anywhere or anything in the game so, like I said, don't act like you own the location either because you don't - so no one is "stealing" anything.