Unless you are experiencing chronic D, I would stay away from Immodium. It is quite common with IBS to alternate between both C and D (sometimes within the same day!) and treating these symptoms prematurely with either something like Immodium or, in the case of occasional C, laxatives can sometimes exacerbate everything.
If you are taking a fiber supplement, you might want to try a higher dose or another brand. If your main issue is loose stool, a bulking fiber should help to absorb the excess fluid in your colon.
Anti-spasmodics are mainly used for, well, spasms. The side effects are pretty nasty and include drowsiness, very, very dry mouth, and C. I won't take them, no matter how great the pain. If you are not experiencing physical pain and discomfort, I would avoid these as well.
You can also include some complex carbs into your diet which might help with bulking the stool. Foods like brown rice, whole grain breads, whole grain pasta, and some of the kashi cereals would certainly help firm things up!