If your 686 kernels work fine, then you can just uninstall the 386 kernels with apt-get remove. I think the package name is linux-image-version-386, but do an apt-cache search searchword to double check. Or use Synaptic.
What I do is load up Adept or Synaptic and search for 'linux-386'. Then I scroll down the list and uninstall all of the 386 modules marked as installed. Keeping the 386 kernels on your computer won't do any harm though. If you have installed a 686 kernel then Ubuntu will use that as default anyway.
