I don't know whether the following comments relate exactly to your condition - but, there are cases described in Medhelp whereby as a last resort for chronic constipation (maybe this is not the same as spastic colon ???) a total colectomy has been suggested. You may wish to ask your doctor whether this surgery could be relevant/helpful to you.
I underwent this operation for torrential diverticular bleeding back in 2004. In response to a question from another medhelp patient I described my experience on:-
http://www.medhelp.org/posts/show/523166
If indeed this operation is endorsed by your doctor (surgeon) you might like to print off the above posting and verify that my own observations mirror your surgeon's expectations. I believe that this operation can now be performed laparoscopically.
regards
Morecambe
PS have you tried to overcome the problem with a high fibre diet rather than chemical laxatives which can lose effectiveness over time.