Raster files are comprised of num_samples X num lines and thus will always be rectangular or square. There is no way to 'remove' pixels outside an irregular boundary but you can mask those pixels and make them transparent. To do this, import the vector to the ROI tool to create an ROI of your boundary. Then use Subset Data from ROIs. This gives you the option to set the pixels outside the boundary to a single value (such as 0). Then you can edit the ENVI Header file, and set the Data ignore value to this background value to make the background transparent.