Meaningless titles such as “this character” have never been allowed. The fact that some of them slipped through the cracks doesn't mean that you should compound the problem. What you're seeing there is more one specific user's bad titling habits. The fact that he used a decent title this time should be encouraged, not reverted.
When I see a title, I need to understand what the question is about, so that I know what it will spoil me about. Otherwise the title is ineffective as a spoiler protection: I'd have to read the question anyway.
The key to spoiler prevention is: the reader needs to know what they'd be spoiled about.
Furthermore, spoiler or not, keep in mind that the whole purpose of Stack Exchange is to produce answers that people will find. Nobody goes looking for “Why did this character do this thing to that other character?”, so such titles make the question impossible to find.
Try to follow DVK's advice: find a way to convey the key elements in the title, and avoid mentioning the spoilery specific event.
This case is borderline. ‘Why didn't Bruce Banner simply tell this character that "I am NOT single"?’ looks really silly, but at least, in combination with the tag, we know roughly what it's referring to. Nonetheless, people who are looking for this question are likely to use “Natasha” as one of their search keyword (together with “Bruce Banner”), so it would be better for the title to include both characters' names.