Select the cell or range with the drop-down list
Go to the Data tab
Click Data Validation
In the Settings tab, click Clear All
Click OK
If needed, copy the cell and use Paste Special > Validation to remove validation from other cells
If the drop-down comes from a form control or ActiveX control, select the control and press Delete
If the drop-down is in a table or named range, remove the data validation from the source cells as well