The Android app maintains a separate list of tags; you can delete them from the app by long-tapping on them.

Hope this helps!

when i long-tap on a tag i see only an option "change the tag". what is wrong?

I'm sorry; I didn't fully specify: once you're in the Edit Tag screen, press Menu and choose Delete Tag.

Hope this helps!

thank you, it works and therefore done for me.

still, adding a "delete" menu item would significantly improve usability at this point, just fyi.