Get unmatched rows for foreign key
A NOT EXISTS can help in this case:

SQL Fiddle

MySQL 5.5.32 Schema Setup:

CREATE TABLE Users(Id INT PRIMARY KEY);
CREATE TABLE Tags(Id INT PRIMARY KEY,User INT REFERENCES User(Id));


INSERT INTO Users(Id) VALUES(1);
INSERT INTO Users(Id) VALUES(2);


INSERT INTO Tags(Id,User)VALUES(1,1);


Query 1:

SELECT * FROM Users AS U WHERE NOT EXISTS(SELECT 1 FROM Tags AS T WHERE T.user = U.id);


Results:

| ID |
|----|
|  2 |


(Thanks, @dotancohen for putting this into a SQLFiddle)