/usr slowly fills

Dear managers

I am finding that the /usr directory slowly fills up:

yoda.vir.gla.ac.uk> df
Filesystem        512-blocks        Used   Available Capacity  Mounted on
root_domain#root      524288      455190       41024    92%    /
/proc                      0           0           0   100%    /proc
usr_domain#usr       4194304     4010042       63808    99%    /usr
/dev/disk/dsk0h     28401404    10038892    15522370    40%    /DBdisk
/dev/disk/dsk1c      8123912     4832428     2479092    67%    /yd1
/dev/disk/dsk3c     69558812    37875490    24727440    61%    /yd2

The usr_domain was 97% full a couple of weeks ago and has gradually 
crept towards 99%.  Whenever it gets to the level that the system 
issues a warning, I delete a few unnecessary things (eg. currently 
unused programs and libraries), but it always seems to creep back 
upwards usually within 2-3 weeks or so.  According to HP support, I 
can't resize this domain as I don't have the appropriate 
licenses.  In any case, making /usr larger would just avoid the issue 
rather than tackle its cause.  Nothing has been installed recently:

yoda.vir.gla.ac.uk> ls -lt /usr
total 368
drwxr-xr-x  14 root     users       8192 Jan 31 11:57 local/
drwxr-xr-x  24 root     system      8192 Dec  5 12:17 opt/
drwxr-xr-x   6 root     webgroup    8192 Aug 11  2005 www/
lrwxr-xr-x   1 root     system        11 Dec 13  2004 news@ -> ../var/news/
lrwxrwxrwx   1 root     system        30 Dec 13  2004 newconfig@ -> 
../../usr/opt/OSP540/newconfig/
lrwxr-xr-x   1 root     system        15 Dec 13  2004 preserve@ -> 
../var/preserve/
drwxr-xr-x   4 root     system     16384 Dec  6  2004 sbin/
drwxr-xr-x   6 root     system     16384 Dec  6  2004 bin/
drwxr-xr-x   6 root     system      8192 Dec  6  2004 lbin/
drwxr-xr-x  22 root     system      8192 Nov 18  2004 include/

and so on....

# du -ks /usr/* | sort -k 1n
0       /usr/adm
0       /usr/dict
0       /usr/man
0       /usr/newconfig
0       /usr/news
0       /usr/pd
0       /usr/preserve
0       /usr/spool
0       /usr/tmp
0       /usr/ucb
38      /usr/users
42      /usr/skel
43      /usr/OV
56      /usr/quota.group
88      /usr/quota.user
555     /usr/cluster
757     /usr/TT_DB
990     /usr/printing
1225    /usr/internet
1265    /usr/tcb
1647    /usr/net
3336    /usr/field
6019    /usr/examples
6220    /usr/lbin
6690    /usr/www
9791    /usr/include
24999   /usr/ssh
35324   /usr/sys
35882   /usr/shlib
51142   /usr/sbin
63263   /usr/dt
76112   /usr/bin
100554  /usr/share
132639  /usr/ccs
229653  /usr/lib
301461  /usr/opt
403975  /usr/var
509109  /usr/local

Any ideas on how to identify what's filling the disks and how to switch it off?

All answers, as always, gratefully appreciated.

Derek