To delete the appearance styles of the signatures, please go to File >Preferences >Signature, click on Appearance Styles then select the appearance style you'd like to delete, click Delete to remove it.
Also, the appearance styles file is stored in "C:\Users\[UserName]\AppData\Roaming\Foxit Software\Foxit PDF Editor\Signature\style.foxitdata".
If you'd like to remove all the appearances at once, you may directly remove this file.
Note: The steps provided above are based on the latest version of Foxit PDF Editor. If you are using an older version, the steps may vary slightly.