The NoData value in USGS DEMs is traditionally set to -99999 which translates to -32767 in signed 16-bit format. There are two ways to ignore this value.
Open the Image Metadata tool (ERDAS IMAGINE Home tab > Information group > Metadata). Select your DEM (File > Open). Display the Set or Clear NoData Value dialog (Edit > Set/Clear NoData Value). You can enter the NoData value in this dialog. If you are not sure what value should be typed into this field you can click the Suggested Value button. Click OK to close the Set or Clear NoData Value dialog and return to the Image Metadata tool.
Create a spatial model to recalculate the elevation values. The model statement can be very simple:
either 0 if ($n1_inputdem < -100) or $n1_inputdem otherwise
Run this model and all of the NoData values will be replaced with 0's.