The fastest way I've found to remove large numbers of formulas is to select the column, copy it, and then paste it over itself using the Paste Values function.
As far as preventing users from repeating this again, the only way I know would be to use the Protect Sheet and Protect Workbook features. Whether these can be adapted to your particular scenario would really depend on what you are trying to protect and what you still need to allow the users to do. Generally, however, you can limit the users to editing only certain ranges, columns or rows by locking all the other cells. There are options to allow users to still insert new rows, if desired. Unfortunately, I don't know of a way to lock the sheets so that users can enter data but can't add formulas.
You could use Data Validation to add an input message reminding the users not to enter formulas. It wouldn't stop them, but it would at least remind them constantly. Data Validation can also be used to force them to enter a specific data type, but it wouldn't stop a formula that provided the correct data type as its result.
UPDATE:
You can explicitly reject formulas in Excel by using the Worksheet_Change event, like so:
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Application.Intersect(Range("C:C"), Range(Target.Address)) Is Nothing Then
If Range(Target.Address).HasFormula Then
Range(Target.Address).Value = ""
MsgBox "You may not enter formulas on this sheet!"
End If
End If
End Sub
In this example, the range checked for formulas is the entirety of column C. You could define any range you want to check, and then blank out the formula, change it to something else, warn the user, etc.
You're right, this can occur when you change the width after creating the dropdown, but also sometimes when there are merged cells present. VBA is the only option. Place this in the worksheet object and change the column number to the column your list is in:
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Target.Count > 1 Then Exit Sub
If Target.Column = 4 Then
Target.Columns.ColumnWidth = 20
End If
End Sub
It will look for changes in your list and adjust to the specified width.
Best Answer
Also, just a side note.... when referring to particular cells in a column (like in your example) you would refer to them as B1:B20. To refer to the whole column, you can use B:B. That describes an array of all data in the B column. Hope that helps.