I think the sort file is written to the /tmp directory or the Directory set by PSORT_DBTEMP. To avoid writing to these directories do the following.
1) You can create new Temporary DBSPACE using onspaces -c -d tempdb -t
-p Path/filename -o 0 -s Size in KB. You should create the filename before you run this command and set it owner to informix and permissions to 664.
2) Add the newly created DBSPACE to the DBSPACETEMP Config Parameter in you ONCONIFG file which is located in you $INFORMIXDIR/etc directory and restart the Engine.