Symptoms

After performing of uninstall of K2 for SharePoint operation according to K2 documentation K2 artefacts are being removed from SharePoint, but there are still SMO's, View, Forms, Service Instances on the K2 side (you can see them in Tester Tool). Can these artefacts be removed as well? Does there exist a stored procedure for this?
 

Diagnoses

The behavior described above, when after performing uninstall of K2 for SharePoint app SharePoint artefacts stay intact on the K2 side is by design and this system behavior is in place to safeguard against the cases when clients accidentally remove their site or K2 app from their site. In such scenario this safeguard measure allows clients to re-add the K2 App and their workflows/views/forms/smo's won't be removed.
This is documented in our K2 for SharePoint product documentation (see 4.6.10 or newer versions) in the very beginning of Uninstall Section:

"Removing K2 artifacts from a SharePoint site does not remove them from the K2 server, it does however remove the remote events which allow Workflows to start."

Sources:
http://help.k2.com/onlinehelp/k2forsharepoint/icg/4.6.10/default.htm_Install_Topics/Maintenance/Maintenance.htm?TocPath=_____8

http://help.k2.com/onlinehelp/k2forsharepoint/icg/current/default.htm_Install_Topics/Maintenance/Maintenance.htm?TocPath=_____8
 

Resolution

In some scenarios it may be necessary to have some supported way of deleting artefacts on K2 side, either in GUI (with some extra checkbox when you do uninstall saying "Also remove related artefacts from K2") or via some stored procedure which can be used by support when clients absolutely insist on the need to delete SP artifacts from K2. At the moment this is not implemented in K2 and there is an existing feature request for this (internal feature request ID 624875).