Not via anything automated, no. You can query tables like sysprocesses, but you'd have to write a job to do so. Also, a knowledgeable user could connect from Excel using ODBC but spoof the application name so it looks like something else other than Excel, so it's not foolproof.
I guess a better way to handle this is (a) why do you need to prevent users from coming in via Excel and (b) why do they have access to do this already (since they could use another avenue just as bad, such as Access)?
K. Brian Kelley
@kbriankelley