Follow these next steps in order to deactivate (disable) Wordpress plugin manually via FTP:
Download and install FTP client (for example, Filezilla).
Open FTP client and enter your FTP access credentials (host, username and password) to access files on your server. Navigate to 'yourwordpresswebsite/wp-content/plugins' directory.
Disable necessary plugin by renaming its folder.
Check the result.
