Its going to be hard to please everyone, if there is a delay when followers are actually being added to the list, then users will be confused or annoyed why the following numbers are out of sync. People generally expect realtime changes, especially on twitter where many people use it to have realtime conversations. People get annoyed when you make them wait 10 seconds let alone 24 hours.
Ideally, twitter could update the cache of the users who the offending account was following.. Not saying its an easy solution, but twitter knows every account that the user was following and looping through those users to remove the spam account in theory would not be too difficult. Quick and dirty solution would be to just invalidate the cache for the users that who the spam account follows, but that would put extra load on their backend.. They do have some method of keeping the list realtime, as when valid users follow or un-follow it is pretty much instantaneous, it just a matter of doing the looping to unfollow everyone that the offending account follows using the existing code path. I am sure someone at twitter has already thought of this, but its not very high priority for them. Someone has to write the script for the abuse team to use to perform this action and I think the developers are probably pretty busy already.
