Есть ли способ запретить модераторам удалять участников?

[HAWK]

Да, единственное решение в вашем случае — это модераторы категории.

[Superceded] Moderation privileges for moderators, category moderators and TL3+ users

Different types of users have different moderation powers in Discourse. The table summarizes who can do what among the main types of power user: Trust level 3, which can be reached automatically or given manually Trust lvel 4, which can only be given manually Moderators, who are ‘staff’ Category moderators who are not staff, but have powers like moderstors but only for the specific categories they moderate Power TL3 TL4 Cat Mod Mod Recategorize topics …