how do you block additions to a drop-down list in Excel 2013

I have an Excel workbook that is shared between several people. One column on the list is a drop down and has different companies listed. I have listed all companies in the source  but I want to block any additions. Employees are able to make an addition by just typing it in instead of choosing from the list and so now we have several duplicates. How can I stop this?
kwhittemoreAsked:
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Subodh Tiwari (Neeraj)Excel & VBA ExpertCommented:
While setting up the data validation list, after adding the source for the list on Settings Tab, go to Error Alert Tab and make sure that the Checkbox before "Show error alert after invalid data is entered" is checked.

Now from the Style drop down select Stop, in the Title box type your convenient title like "Invalid Entry!" and in the Error message box type your message which will be popped up when a user tries to input a value which is not in the drop-down list like "Choose from the Drop-Down List only." and click on OK.

Hope this helps.
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Microsoft Excel

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