Open Logic.
Click and hold the "Input" button to see a list of your software synthesizers. Click the synthesizer containing the orchestra instrument settings that you want to delete. For example, click the "ES 2" synthesizer to access your "French Horns," "Brass Section" and other settings. Click the "ES E" synthesizer to access your "Bright Strings," "High Strings" and "Synth Brass."
Click the "Media" button in the upper right corner of your "Arrange" window (your main project window, which you first see upon opening Logic) and click the "Library" tab to see a list of your instrument presets corresponding to the currently selected synthesizer. In the first of two columns is a list of instrument folders, separating each preset according to bass, drums, brass instruments and other settings. When you click on any folder, you see a list of the corresponding instruments in the second column. Click the instrument that you want to delete.
Click the instrument window, which features the name of the current software synthesizer (such as ES 2, ES E or EFM1) and a series of knobs, with which you can edit the sounds. At the top of the window, notice the name of your current sound setting. For example, if you're using the ES 2 synthesizer, you might see the words "Philharmonic Strings" along the top of your window. Click the title once to access a drop-down menu.
Click "Delete Setting" from the drop-down menu.