To exclude files from sbt one can use excludeFilter:
excludeFilter in unmanagedSources ~= { _ || "shellscript.scala" }
which is equivalent to:
excludeFilter in unmanagedSources <<= (
excludeFilter in unmanagedSources) {
(currentFilter: FileFilter) => currentFilter || "some-file.scala"
}
)
More on this can be found in the online documentation for sbt.