You can't do that, but you can 'deauthorise' all your computers, then re-authorise the ones you still want to use. To do that
Go to itunes
go to the itunes store
under 'quick links' on the right, click 'account' - you may need to put your itunes store password in at this point
Under there somewhere, there should be a 'deauthorise all' button.
To then authorise a computer
open itunes
select 'authorise computer' from the 'store' menu and type your password in.
Note if you do this part on a computer you've already authorised, it will at least tell you how many computers you've currently got authorised to the iTunes account.
Jon