Currently when you reply to the employer, it asks you to select interested or not interested. When you select not interested, a number of reasons comes up, one of which is "This position does not appeal to my interests at all". Each of the not interested reasons carries a specific weight and we monitor the overall performance of the messages each employer sends. On occasion we have used this information to encourage the employers to write better and more personalized messages.
If you feel the employer is not someone you would want to work for ever, there is also an option to block the employer from sending you any more messages when you reply not interested. There is also an "other" not interested reason which allows you to type a message that only we will see.
In addition, we will soon be rolling out an update which should prevent most occurrences of employers sending messages to candidates with the wrong name. Part of that was a technical goof.
EDIT: To clarify some things brought up in comments.
First off, we are all developers here and are working to eliminate exactly these types of problems from the job seeking experience. We have a passion to avoid as many of the annoying things which make people hate looking for programming jobs as possible. In our eyes, allowing spam has a negative user experience value. We believe that our revenue increases and decreases by our user's satisfaction. All of our users, not just those users who write the checks. They are only writing checks because they expect good candidates to be there, which they wouldn't be if we allow all of the annoying things.
The points brought up in the original post regarding the quality of the message (canned messages, nothing to do with your profile, someone else's name) are all things we are working to eliminate. That being said, we're not in the business of writing an employer's messages to their selected candidates. If we did, you'd end up seeing a bunch of... canned messages. We know what makes a good message and we encourage employers to use those tactics. However, it's more important for employers to use those tactics because they think those tactics lead to more successful hires than to use them because the system forces them to do what we think is best. Hey, we're wrong sometimes.
It is true that not being interested in a position for an ideological problem doesn't have anything to do with the quality of the message. That's why we don't go slapping employers on the wrist all willy nilly when their response score is low. We look into their situation and make a determination on a per employer basis.
We added not interested reasons as a way to help increase the quality of messages coming from employer on site. A negative value for candidates would be having to sort through a list of 10 reasons why you aren't interested an think about the differences between them in order to make a selection. We want to keep the list short. If you have a specific reason that you don't think is appropriately handled by one of our pre-filled reasons, submit an "Other" response with a description. Once enough similar submissions come in for other reasons, we'll be quick to add it as a selection.