How to remove certain UML labels in Visio 2010
While the statement in the answer above is true it is not the solution to the problem. Most of the UML shapes have a wide variety of display options. To remove the decorations on a UML connector do the following:
right-click the shape
Select "Shape Display Options..." from the menu
Un-check End Options ' "First end name", "Second end name" and "End multiplicities"
(Alternative) In this case you might select Suppress / "First end" and "Second end". Checking the "End Options" turns things on; checking "Suppress" turns things off.
Play around with the other UML shapes to see how they behave.
That label is probably grouped with other items and cannot be removed.
See if this Microsoft article helps :
Visio: Cannot Ungroup or Modify an Object in Visio