Hi Peter,
you can use this dms-service endpoint to deactivate a process model. Please replace the {modelId}.
/model/{modelId}/deactivate
You can find a description here, but notice this is an internal endpoint, so you have to check the box on the end of the page:
http://cios.optimal-systems.de/rest-ws/#ENDPOINT:BpmManagementService.deactivateModel
The model should be now deactived. This means you cant start new processes. But may be this means too that you cant delete it. Because you cant delete models, if there are running process instances. So, before you can delete the model, you have to ensure that no running processes are exists.
The followings will be helpful:
http://cios.optimal-systems.de/rest-ws/#ENDPOINT:BpmProcessService.getProcesses
http://cios.optimal-systems.de/rest-ws/#ENDPOINT:BpmProcessService.deleteProcesses
For the first endpoint use the modelId parameter to restrict the results.
I hope this helps you. Feel free to ask, if you have some further questions.