getting rid of unwanted field entries

I have a collection of about 500 albums, which is fairly well sorted and organized. There are a few programs though that seem to ignore this. iTunes and Media Center come to mind. What happens is this: Some years ago I decided to use the "interpret" field only. "album interpret" is empty, at least when I look at it with MP3Tag. However, iTunes and MediaCenter sort the collection based on album interpret anyway, somtimes with odd results - sometimes on half of an album is "various artists", the other half is "various". It drives me nuts!

The only explanation I have: Either not every "album interpret" field in every tag version is empty or these programs collect additional information from the internet. What would be the best way to prevent such behaviour? (And no, deleting every tag and starting from scratch is not a preferred option).

Have you checked the files with "View > Extended Tags" ?

Yes. Problem right now is: I think that I cleared all the fields. I'm not really sure - I had to reinstall windows and after reinstalling itunes afterwards, some of the fields are back there. Is it documented that itunes or other programs load tag data from the internet or am I just seeing things?

WMP and itunes have their own databases which are updated in more or less unpredictable intervals. So using them as a reference is not such a good idea.
But you are on the right way: let mp3tag do the tagging.

mp3tag ist a powerful tool with even large libraries. So why don't you load all your tracks into mp3tag and filter all those
NOT %band% IS ""
(I am not sure whether you used a field "album artist" or the mp3tag equivalent %band%)
Then: clearing does not mean to empty the field but you really have to delete the tag by setting the contents from to .

And yes: there are some settings in WMP and itunes that allow these programs to retrieve information from the internet. But you can and should switch it off.