I have had someone access my account, play a few games, changed my settings and spend some in game currency. I've changed my password, buy how else can I prevent this from happening? How were they able to in the first place?

Back when I was playing competative matches to boost my rank. I got into a match, it was ascent, and chose Omen. There were a couple of duelist and I forgot who the rest are, but that doesn't matter. These duelists kept going mid as a flank...