The following commands, copy/pasted into /Applications/Utilities/Terminal.app, should inhibit the warnings:
defaults write net.tunnelblick.tunnelblick skipWarningAboutBigSur1 -bool yes
defaults write net.tunnelblick.tunnelblick skipWarningAboutBigSur2 -bool yes
defaults write net.tunnelblick.tunnelblick skipWarningAboutBigSur1m -bool yes
defaults write net.tunnelblick.tunnelblick skipWarningAboutBigSur2m -bool yes