When using Undrained B materials in PLAXIS 2D 2010 it is not possible to activate or deactivate interfaces in the “water conditions” mode to set boundary conditions for groundwater flow calculations.
Work around is to go to "staged construction" mode and temporarily appoint a different (drained or Undrained A) material to the relevant cluster(s) and go back to "water conditions" mode and activate or deactivate the relevant interfaces. After this, appoint the original material set to the relevant cluster(s) in "staged construction" mode.
This has been fixed since PLAXIS 2D 2011.00