Depends on the plug-in.
There is no one answer except reinstall should get the factory settings back. Some plug-ins make it difficult to step on factory presets, others provide no warning.
Different vendors store presets differently. For example, Nomad stores presets in "My Documents\Nomad Factory\Plug-in Settings" in plug-in specific folders; Sonitus stores presets in the registry.
The Cakewalk Plug-in manager provides a way manager presets stored in the registry. Generally user presets (fxp) and banks (fxb) may be stored anywhere.