Now, that's a bit more difficult!
You can filter the MyFlights list, but only by one fleet at a time.
Flight ID <> BMA*
Flight ID <> BAW*
Of course, they will still show on the map.
You can still use the Fleet Watch - Show only Fleet Watch Aircraft, but you will have to have all the other fleets in the list, like:
VIR*
EZY*
DLH*
etc
In other words, everything except the fleets you already have. This will be a longish list, but it will work. Just watch the MyFlights list and add operators as you go along to build up you list.