You can create a Python script that makes use of Dataiku Python API to delete multiple projects at once. Below is an example code that can be used to delete the projects corresponding to the project keys defined in the input list project_key.
apiKey = "*************"
client = dataikuapi.DSSClient(host,apiKey)
dss_projects = client.list_project_keys()
for key in project_keys:
if key in dss_projects:
project = client.get_project(key)
print("Project" + key + "deleted")
Please note that delete project call requires an API key with admin rights.