What do you mean 'from within a step'? What kind of step? For a Run Script step or a step script event, set Step.BuildStatus = vbldStepStatFailed or raise an error (Err.Raise in VBScript or throw in JScript).
http://www.kinook.com/VisBuildPro/Ma...riptevents.htm
Configure the step to build all or a specific failure subroutine on failure, and if the step fails, the specified failure steps will be built and the build will stop.
http://www.kinook.com/VisBuildPro/Ma...tepfailure.htm
http://www.kinook.com/VisBuildPro/Ma...restepstab.htm