One of the items on our backlog is the ability to set a retention policy for projects, which automatically cleans up old deployed applications, cached NuGet packages, and releases/deployments from the Octopus UI.
This item isn't done yet, but in the mean time, if you need to delete releases/deployments, you can now do it using the latest version of Octo.exe along with an Octopus server running version 1.0.31 or higher.
From the command line, the syntax is:
octo delete-releases --project=MyProject --minversion=1.0.0 --maxversion=2.0.0 --apikey=ABCDEF... --server=http://<your-octopus>
The version numbers are inclusive, and can be partially specified - e.g.,
2.0.1982.12981. SemVer rules are used when comparing the version numbers.
As I said, this isn't enough to be a full retention policy feature, but it might useful if, for example, you accidentally created a few hundred releases because of a circular dependency in your build/deploy scripts :)