Hello again,
I just found how to do so in the link you sent. Thank you very much!
One simply has to enter <extra></extra> at the end of the string given to the hovertemplate. As it is said in the documentation:
Anything contained in tag
<extra>is displayed in the secondary box. […] To hide the secondary box completely, use an empty tag<extra></extra>.
So in my first example, it can be removed with: hovertemplate = "Sample %{customdata[0]} <extra></extra>".
Thank you for your help!