How much to trust them?
Really, not at all.
Several possible solutions come to mind:
- For the openly insubordinate, fire them for cause.
- For those whose actions do not rise to that level of offense, hedge in their worst actions with official rebukes/warnings. Reduce that section's personnel, as any department that has so many people that they can waste time plotting against their boss needs to experience some trimming of the fat.
- For those who you want to fire for cause, but not fire-able, transfer them to remote outposts, with PLENTY of busywork - and deadlines - to keep them occupied. Do not let them have subordinates to re-build their empire. Do not let them have ANY public opportunity to speak officially.
Ask for recommendations for staff cutbacks. Don't take their advice.