Denny has a point - if they can see the data they can make a copy of it if only by typing it themselves into a spreadsheet.
However, you can make it more diffcult for them to do this wholesale by limiting all user database access to stored procs only which do not ever under any circumstances use dynamic SQL (which requires you to give rights to the tables themselves). Then you give them the rights only to the stored procs and not the base tables. This way they can only do what the stored proc specifies and nothing else. At least nothing directly to the data.
If you are a shop with legal requirements that the data is private or if you have confidentiality issues with the data then you need to do this and have every employee sign a paper that tells them that they acknoledge the data is private (and/or corporate property) and is not to be used outside the existing database or taken off site or put into another database or used for private purposes or sold, etc. A lawyer should write this agreement. You should also consider if the dat should be encrypted in the the database and only decrypted by the GUI. One of the most common ways that data is stolen is from the backup tapes which are usually unencrypted. If the data itself is encrypted then even if it stolen, it won't be directly legible.
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