Buffy probably knew Spike wouldn't be doing it again. And it was not like one season earlier, when he was just annoying her to death with his feelings, they actually had something this season. Even though Buffy never wanted to admit it, she was somehow attached to Spike, and I guess that's how. And it wasn't like with Angel when he turned into Angelus, because Spike was still no risk for others because of his chip. And well, most of the time Buffy could handle herself.
On top of that, let's look what happened after 'Seeing red'. We had Willow going evil and getting lost in magic, and then her in England. By the time she was back and could've performed a spell it was all more or less forgotten, plus Spike was crazy in the basement, so no real threat anymore.