According to the Spock website:
Spock provides a JUnit runner, and this is what allows it to be compatible with IDEs and build tools without explicit support for Spock in those tools. So, presumably, to run Spock tests from sbt, you'll need junit support. Simply add:
"com.novocode" % "junit-interface" % "0.11" % "test"
To your projects library dependencies, and you'll have JUnit support in sbt. Then, I'm guessing, Spock should just work, but I haven't tried it so I got no idea. For more info on the junit support in sbt: