There are two ways that you can accomplish this using the NI-488.2 API.
First, if you are using the 488.2 style calls, you may use the EnableLocal function. It takes the boardID and an address list as its parameters.
Otherwise, get an interface handle to the GPIB board(not device). Then issue an ibsic call to ensure that the board is CIC(controller-in-charge). Following that, issue an ibcmd call. The buffer to give to ibcmd should consist of the following:
UNT(untalk) -- 0x5F
UNL(unlisten) -- 0x3F
MTAx(make talk address the PAD of the board) -- 0x40+PAD
MLAy(make listen address the PAD of the oscilloscope) -- 0x20+PAD
GTL(go to local) --0x01
Essentially, that sequence of command bytes does the following: Unaddresses all instruments on
the bus(UNT,UNL), addresses the GPIB board to talk(MTAx), addresses the oscilloscope to listen(MLAy), and forces the listening instrument to local(GTL).
For more information on the IEEE 488 Command messages, refer to the NI-488.2 online help.