As ITnavMan man says you navigate to Administration > Solutions > Plug-in Management. However there is one gotcha here in that even if you are logged in with administrator privileges you won't be able to see this. Instead you need to login either as the SSO administrator (admin@system-domain) or as a user with those same privileges (SSO Administrators group)
You can disable plugins here but clearly be very careful!
For example you can disable the vCenter Orchestrator plugin and once the vSphere Web Client has been reloaded all traces of Orchestrator disappear from the UI.
Now this is all well and good but as others have said there isn't a VUM plugin for the vSphere Web Client......