This scenario reared its head when it looked like all of the recourses within the offending namespace were deleted but the actual deletion of the namespace had hung.
Firstly try to delete forcefully with:
kubectl delete ns ns-example --force --grace-period=0
If this fails...
Check for active finalizers if a namespace is failing to delete:
kubectl get namespace ns-example -o json
If so, we can empty it with:
kubectl get ns ns-example -o json | jq '.spec.finalizers = []' | kubectl replace --raw "/api/v1/namespaces/ns-example/finalize" -f -
The app deletion in ArgoCD should now have (hopefully) been successful.