Can hosts terminate a challenge?
Once the Challenge has started, the host has no way of stopping it.
Host can stop receiving further entries by reducing the maximum number of entries to the number no lesser than the number of entries already collected, which will merely stop further submissions.
Even if the challenge was blocked from accepting the entries, it will run its course, meaning that the challenge title will remain visible for the rest of both (submission and voting) phases.
I presume DPR can remove it from the run, but the next in that host's series, the one already announced, would wait for its submission date to open it for entries. Best ask DPR by Feedback.
As to the unqualified entries, (I presume, these would be the ones not conforming to the proposed theme and the rules), the host should simply DQ them, even if that means only one correct entry in the finals.-- hide signature --
DQ unqualify entries even if that results into a challenge with no entries at all.
"Ethics is the difference between what you have the right to do, and what is right to do."