The directories below are the default locations for where the presets are saved -- if you moved the presets to a custom location, go there and then follow the instructions below. 

To delete a preset:

• On Windows, go to: C:\ProgramData\GenArts\Sapphire\presets\GenArts
• On Mac, go to: /Library/Application Support/GenArts/Sapphire/presets/genarts/

Select the preset you want to delete. (They are in this format: S_PluginName_PresetName.gp.)

Right click on the preset file and select "Delete".