Hi,
The HTML panel is set for the design of the emails and will not affect the plain text only property.
However, if you prefer to get rid of it anyway, please create a new designer feature protection in the Settings of the Marketing application.

Select the Team or User whose HTML panel you' d like to block and tick the HTML in the drop list of Blocked features.
The HTML panel will not be shown for the user/ team after save it.

If this helped you, I'd appreciate it if you'd mark this as a Verified Answer, which may in turn help others as well.
Best Regards,
Nya