If the posts from the ignored user is harmless, ignoring the user would suffice imo. If it's not, there's always the report function.

I suppose you could politely tell them to stop posting in your threads. If you simply ignore them on your threads, they might eventually come to the realization. The former option is quicker. But my intuition is that directly confronting them is more likely to elicit... emotionally charged responses. It might be politely worded, but in the end you're still telling them to stop contriburing ro your threads.

Or.. they could ask you "why am i on your ignore list? ;( " and you explain yourself, to which they respond, "I will change my ways!! :* and everyone lives happily ever after

I may have stayed awake too long