Don't know if it is the only possibility, but just change the group or location to client control:
Clients > [Group] > Policies > Location-specific settings
By default, the group is set to Server control. If you change it to Client control, you can terminate/block etc. in Network Activity window.
However, Client control is a dangerous setting because you will transfer the complete Firewall and IPS responsibility to the user, which is not wanted in 99% of the cases.
You can block applications in the SEPM firewall policy as well.