How to restrict other picklist values getting saved in particular picklist field other than its predefined values in salesforce?

Solution : By selecting " Strictly enforce picklist values" checkbox, we can enable validation of picklist values against the list of defined values.
we define picklist values when we create the custom picklist field.
However, users can introduce extraneous/new values by loading values through the API.
In order to restrict adding new values to picklist through API's,We need to select "Strictly enforce picklist values" checkbox when creating/updating the custom picklist field as shown in the image below.

Note : Only a new custom picklist field can be a restricted picklist. we can't convert an existing non-restricted picklist to a restricted picklist.