Hello TECHIT SRIWICHAI,
My understanding is that you can create a DLP policy in Microsoft 365 to prevent users from downgrading sensitivity labels
Can you please check this video tutorial and see if it helps.
Other reference documents:
https://learn.microsoft.com/en-us/purview/dlp-learn-about-dlp
https://learn.microsoft.com/en-us/purview/dlp-policy-reference