Hi,
The VersionService provides both deleteVersion and deleteVersionHistory. You can write a java backed webscript or write a custom Java Action that calls such methods.
You can write something like the following in your java class
VersionHistory history = versionService.getVersionHistory(nodeRef);
for (Version version : history.getAllVersions())
{
if (version.getVersionLabel().equals("3"))
{
versionService.deleteVersion(nodeRef, version);
break;
}
}
Hope this helps.