Imagine a world in which technology we would probably call "[technological singularity][1]" is possible and already happened on few places of the galaxy. By the singularity I do not mean that anything is possible, but rather that original biological species transform themselves into extremely highly intelligent computer programs with big amount of control of their environment, and they completely transcend their biological nature. The post-singularity societies might rebuild planets to giant computers or even create [Matrioshka brains][2], since the main scarce resources are computer time and memory. It would be relatively easy for such civilization to send spaceships that would transform all solar systems in our galaxy into computers and utilize them. It would be even easy for a small splinter faction inside such civilization, unless the "official forces" actively prevent it. The motivation is simply a lot of resources and freedom to gain. 

My question is: *Are there plausible reasons that would prevent it and keep the civilizations bound to their home star systems?*

If there is such, the reason should be strong and  valid for all such civilizations. If nine out of ten civilizations decide not to spread, and one does, the galaxy will still be transformed. 

[1]: http://en.wikipedia.org/wiki/Technological_singularity
[2]: http://en.wikipedia.org/wiki/Matrioshka_brain