Since you've got both the shell that you're typing into and the shell that sudo -s
runs, you need to quote or escape twice. (EDITED fixed quoting)
sudo -su db2inst1 '/opt/ibm/db2/V9.7/bin/db2 force application \(1995\)'
or
sudo -su db2inst1 /opt/ibm/db2/V9.7/bin/db2 force application \\\(1995\\\)
Out of curiosity, why do you need -s? Can't you just do this:
sudo -u db2inst1 /opt/ibm/db2/V9.7/bin/db2 force application \(1995\)