Adware-Onestep.b has been around for a while and McAfee knows about it. According to this information from the Threats Database it should be removeable provided you've got an up-to-date DAT file containing the latest virus definitions.
From what you and others say, the scan does detect it but does not automatically delete it. In that case, does the scan give you any options as to what to do? According to the Help pages,
When a scan is in progress, your scan page shows which files are being analyzed, reports issues as they are found, and displays the fixes that take place. If you want to cancel the scan before it finishes, you can still resolve any issues that McAfee discovered.
When a scan is finished, you can view the results for infected items and potentially unwanted programs. Scan results include the exact location on your PC for each infected item, and the names of the security threats it contains. You can also view the details of potentially unwanted programs, including a full list of affected files and registry keys, which contain information and settings for your operating system.
No-one has yet told me exactly what happens with adware.onestep when the scan finds it, so I can't say whether the program is behaving as it should. If McAfee classes this as a Trojan, then the scan should remove any malware and fix any infected files.
Of course, if the Trojan has mutated it may be that there is a different set of infected files, and maybe that is why the removal process cannot be completed. In that case, if you send an infected file to McAfee (the file used to be ckz_7lro.exe, but that may be different now) then the Labs will analyse it to see what's changed. The place to submit a sample of malware is https://www.webimmune.net/default.asp.