Is it possible to spell a word to Siri? For instance, if I ask for the stock price of a certain company, she doesn't always understand the company name.
It doesn't work reliably but there are a few commands for spelling things out in Siri. Here's an article about it: http://www.tuaw.com/2011/10/20/how-to-type-comma-using-siri/
It sounds like some Dragon Dictate commands work.
You can say
no caps on and
no space on and then spell a word, letter by letter, followed by
no caps off and
no space off and Siri will understand that you spelt a word out for it to interpret.
You can see this in practice in this video: https://www.youtube.com/watch?v=xMNLV-qeou4
The context of the video is spelling words in Notes.app, so it's not clear if this works in all Siri contexts.
I'm afraid that's not possible. You can't force Siri. At least, not yet.