I can think of three options - Choose the one that suits you best:
1) Force off all applications and connect to the database in exclusive mode
$ db2 connect to sample in exclusive mode
Once you terminate, the database will accept connections.
This will prevent new applications connecting to the database.
2) If all clients are remote tcpip clients, then update the svcename to '' , stop-start the instance. So there will be no listner and hence the database will not receive any remote requests. Once you have your job done, change the svcename to the correct value and stop-start the instance.
3) If you have some local clients also, you can probably revoke just the CONNECT Privileges for the group of users before you start your work. On finsihing your work, re-grant the privileges.
Cheers
Sathyaram