Try to delete with force removal option.
If it's not possible from the UI, you would need to use REST API and set the forceRemoval=true
NSX API Guide - http://pubs.vmware.com/nsx-63/topic/com.vmware.ICbase/PDF/nsx_63_api.pdf
If you would like to do it yourself but new to REST API,
1. check these blog post:
2. check this document: Automation Leveraging NSX REST API Guide
3. try VMware Hands on Lab VMware Learning Platform HOL-1725-SDC-1 - VMware NSX Advanced Consumption
or open a VMware Support Request if you have an active support & subcription.
Bayu Wibowo | VCIX6-DCV/NV
Author of VMware NSX Cookbook http://bit.ly/NSXCookbook
https://github.com/bayupw/PowerNSX-Scripts
https://nz.linkedin.com/in/bayupw | twitter @bayupw