If you could do this with BOINC v.6 but can't with v.7, you could ask on the PrimeGrid forum a) whether trying to do this is a good idea and if they say yes, b) whether anyone knows how to do it with BOINC 7.

If the BOINC 7 code isn't right for PrimeGrid for this feature, Rytis needs to know. He and the PrimeGrid moderators will only know if you tell them on their forum.

If this feature worked with BOINC 6 but doesn't with BOINC 7, you could in any case go back to using BOINC 6. I am still using 6.12.34 and am very happy with it. If you want to use BOINC 6 again there is a place to download the older versions from.

In the BOINC Manager Preferences box, in the Network usage tab should Crun-chi set 'Connect about every ___ days' to 0? Would this prevent BOINC from downloading new work until it has completed the task it was processing?

In the BOINC Manager Preferences box, in the Network usage tab should Crun-chi set 'Connect about every ___ days' to 0? Would this prevent BOINC from downloading new work until it has completed the task it was processing?

I think that I repeat in this post at lest few times that all buffers are set to 0 :)
In short: no, this will not "disable" Wu to start download when old one goes to 80% of progress...

So please, is there any option, to disable downloading new WU until old is 100% finished?

This question is pretty clear, I hope?

Yes, but you didn't say why you need such strange behaviour.
((almost) nobody can know there exists strange project which awards only the first returned task.
And is this true I still don't know (I have to check at the project's site but I'm not interested in a project which do something (award only the first) that no other BOINC project do).
)

If you ask "How can I cut my leg" everybody will say "But why? You can just wash it" ;)
Until you make it clear "I have cancer in the leg"

In my country, when I ask clear question, I got clear answer :)
Why I need something is my "problem" question was very clear and simple, as answer , yes or no :)

You are very lucky to come from that country.