Personally, what ruins roleplay for me is the following:
~When they have loads of characters but no personality.
~When their characters are overpowered.
~When they rip things from other franchises.
~When we're in my world and their characters don't suit my world's rules, even when I've reminded them of the rules (such as law of magic, or the fact that tokyo ghoul's ghouls don't exist in my story).
~When they're presented as mentally ill, but they don't present it properly. The most common examples are Yanderes that know they're Yanderes. Stop. They do not know that they are Yanderes.
But what really ticks me off is when they're literally ABUSING my characters. I remember on my first roleplaying platform, my character suffered from extremely severe mental illnesses. But his lover would just use them against him! My character used him as a sex slave and didn't properly take care of him, but to make it worse, would purposely exploit the mental disorders he knew he had, such as threatening to break up with him, or joking about cheating on him with his brother, despite him having borderline personality disorder, and he KNEW that! If you're going to have your character be in a proper relationship with somebody, you need to heed to their needs and not use them against him!
People like that are the worst, in my opinion...