Creating a WEBID and setting its privileges only affects panel access. To limit a particular FTP log-in to one domain only you need to place that domain under a different machine/FTP user. It sounds like all your domains are under the one machine/FTP user.
Create a new machine/FTP user in the panel (Users -> Manage Users -> Add New User), then place the domain under the control of that user (Domains -> Manage Domains -> Edit).
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