How to delete/map Activities?

[Petrus de Calguarium]

I have plasma set to use 4 desktop activities, one for 
each virtual desktop.

For some reason there are 6 activities (why?) and at 
each login it can, and occasionally does, happen that 
a different activity is mapped to a virtual desktop. 
The configuration will stay for days, even a couple of 
weeks, and then, all of a sudden one morning when I 
boot up, an activity will appear on the 'wrong' 
virtual desktop (not a big problem) and one of the 
unused activities will appear on a virtual desktop 
(definitely a nuisance).

How can I delete the undesired activities (there is no 
delete X when I zoom out)?

Why does the one appearing on virtual 1, vt 2, etc. 
keep changing without any action on my part?