You've got it, you should be able to just enter that port-channel 128 interface, and then issue the no spanning-tree command.
Its a real hassle to sh run on one of my FX2 IOA switches.  Basically it shows every single vlan it seems.  I can't even get ot the bottom of the config.  An example is that every vlan shows something like this:  interface Vlan 1735 mtu 2500 no shutdown   I just want to know if the port channels I have uplinked to our Cisco 3750x switch have spanning tree on them or not.  We are migrating to an mlag across two Extreme Networks X690 switches and any switch connected to the MLAG needs spanning tree disabled.  That is simple on the Cisco access switch portchannels and member ports, you just enable bdpufilter on those interfaces. I am trying to wrap my head around the Dell FX2 chassis IOA configuration and its very difficult.  It doesn't even seem to have tab completion or command help with the ? at the end of the commands.  Basically I need the two 10 gig sfp+ fiber links in an LACP trunk that continues to carry our vlans 1 through 20.  I don't need all 4096 vlans in existence.  Thank you for the assistance!
Why does one of my FX2 IOA switches show every single vlan in the config