Here in the printer’s manuscript, Oliver Cowdery initially wrote a multiple negative, “that they should not constrain no man to be their king”. Almost immediately, Oliver crossed out the not (there is no apparent change in the level of ink flow for the crossout). Although the original manuscript is not extant for this part of the clause, spacing between extant portions argues that there was no not in 𝓞 (unless it was supralinearly inserted).
As explained under Helaman 13:28, there are no clear examples in the manuscripts where Oliver Cowdery permanently created a multiple negative. In every case where the original text appears to have had a multiple negative, Oliver wrote it down in 𝓞 and copied it into 𝓟 without correction. Nor is there any sign of him editing out multiple negatives that were in the original text—unlike the 1830 typesetter, who sometimes did (see the examples listed under Helaman 13:28). Thus here in Ether 6:25, the critical text will accept the corrected reading in 𝓟 without the not (and in apparent agreement with the reading in 𝓞).
Summary: Maintain in Ether 6:25 the corrected reading in 𝓟: “that they should constrain no man to be their king”; spacing between extant fragments of 𝓞 argues that there was no not before constrain in 𝓞.