Is there any way we can exclude all possible values ?

hi all,
   Is there any way we can exclude all possible values to be  assigned to a 
column in a table  without knowing the type of the column ?
   For example,suppose the name of the column is COL,if the type of the column 
is number,perhaps we want to return COL =a, since the type of the column is 
number,so the value to be assigned to it can not be 'a',through this we can 
exclude all the possible value.
   Though this seems to be queer, but in our project, we have this need.
   
   Can anyone tell my how to do?

   Thank you in advance
   Best Regards

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