Hi Vito, the presets are stored in *.dat files in the C:\Skyline DataMiner\users\[username]\Spectrum Presets folders. There, you can find the user who created the preset and who will be able to delete this preset.
Suppose this user no longer exists in your environment. In that case, an administrator can remove the files manually, but be aware that if you remove files from the file system a DataMiner restart will be required to reflect the changed files correctly.