Hi,
Follow below steps.
Create Var1==Substr(Right([Role];(Length([Role])-Pos([Role];":")));1;3)
Create Var 2==Sum(If [Var 1]="MSS" And [System]="BW" Then 1 Else 0) In ([User])
Create Var3==Sum(If([Var 2])<>0 And [System]="ECC" Then 1 Else 0) In ([User])
Apply the filter on where Var 3 equal to 0.
With above contions you only get the users where no MSS role in BW.
I have tested with dummy data and it is working .
Regrads,
Amit