I am using Foxit Reader version 126.96.36.1995 and “Manage Signatures” only allows me to add a signature, not delete a signature.
I finally figured it out. The manual says
To manage the signature, do the following: Choose PROTECT tab > Protect > PDF Sign > Manage Signature. In the Manage Signature dialog box, click Create to create a signature. Select a signature you created, click Edit and you can change your signature. You can preview the signature after you edit it. To delete a signature, select the signature and click Delete.
I couldn't figure out how to select because the various signatures I had created weren't in a dropdown list, or separated by anything. Nevertheless, it turns out they are selectable:
In Windows first find the file with the signature you want to delete. For this you need to open a command prompt (black screen):
dir %SystemDrive%\ /b /s | find /i "InstaSign" | find /i "UserSign"
Then open that directory and delete the signature file. Signatures are PDF documents that you can open with Foxit itself. Just delete what you want.
If you want to search only within your account use this version:
dir %HOMEPATH%\ /b /s | find /i "InstaSign" | find /i "UserSign"
As far as I know, for advanced editing you need FoxitPhantomPDF.
I have phantom standard and had a signature I no longer have the password for. It took me forever to find the "manage signature" spot, but to delete it I still needed the password. I found the file in appdata>roaming>foxit software>foxit PhantomPDF>Instasign>UserSign. From here I could delete it without a password. Of course, this is windows based directions, but I imagine Mac has a similar user file that probably very very hidden because they don't trust their users at all. Maybe something to do with dragging the harddrive to the trash.