Numbers floating sheets
In your question, you mention:


  double tap


So is it safe to say you are referring to Numbers on iOS?  If so, to lock a table or element do the following:


Tap the Format icon (Paint brush)
Tap the Arrange tab.
There you will see a Lock.  This will lock the element from moving and prevent editing.


To edit again or move, unlock in the same manner.

Hope this helps!