list1 = {{1, 2, 9}, {2, 3, 7}, {5, 6, 9}, {8, 10, 11}};
list2 = {{1, 2, 9}, {8, 10, 11}};
Complement[list1, list2]

It is unclear what you're asking, but perhaps this is your solution:

list1 = {{1, 2, 9}, {2, 3, 7}, {5, 6, 9}, {8, 10, 11}};
list2 = {{1, 2, 9}, {8, 10, 11}};
Complement[list1, list2]