I'll let others with more experience about this topic chime in however in case you haven't already seen the documentation. Here is some additional information on Profile attributes
Mihnea Docea | Technical Support Consultant | Customer Experience | Adobe | (:: 1 (800) 497-0335
@Jennifer, The profile attributes are not overwritten by default. Unless you explicitly change the value, it remains the same till the profile expires. Hence you will need to pass a new value into a profile attribute and then previous value is overwritten and is no longer associated with that attribute.
Hope this helps answer your question.
Just echoing what Shruthi said, currently you will need to wait for the attribute to expire.
However, there has been a feature request tied to this, here: Delete profile attributes
Would be great if you vote up for it as well!