With respect, the answers the others gave are not completely true. You can force off individual users for one database in a single statement.
Example: Let's say you have 2 databases on one instance - db2_1 and db2_2. You want to force off only those users on db2_2.
db2 list application:
DEVX java 336 xxxxxxxxxxxxxxxxxx DB2_2 5
DEVX java 309 xxxxxxxxxxxxxxxxxx DB2_2 1
DEVX java 291 xxxxxxxxxxxxxxxxxx DB2_2 5
DEVX java 334 xxxxxxxxxxxxxxxxxx DB2_2 5
DEVP cnr64 415 xxxxxxxxxxxxxxxxxx DB2_1 5
PSADM ppagent 160 zzzzzzzzzzzzzzzzzz DB2_1 1
PSADM ppagent 159 zzzzzzzzzzzzzzzzzz DB2_1 5
OK, so users 336, 309, 291, and 334 need to be forced off. So:
db2 "force application (336, 309, 291, 334)"
You must use the quotes or the command will not work. Maybe this is why your previous attempt to do this failed? You can also set this up in a script that looks for the users for the database in question, then creates the statement for you and runs it.
Hope this helps.
-- Steve