Microsoft Word: Uninstall Secured Signing Plugin on Microsoft Word Online

Step 1: Go Office Add-ins popup

In Insert tab > Click Office Add-ins button to load Office Add-ins store.
In My ADD-INS, you will find Secured Signing for Word, mouse hover it, then click the 

Step 2: Confirm removing action

Click Remove menu item.
Confirm to remove by click remove button in the popup
Click Refresh link to force plugin to be cleaned from current environment.

Do you need anything else?