Prevent others from adding delete or other characters to the QueryString to delete your database contents

Now some people on the network have to use Programmer's SQL Programming Vulnerability (estimated and called vulnerability: P)
After QueryString, add a statement such as:;d elete forum_forum;--, to delete your database.
The reason for this is that programming is not tight. I give you a small way, perhaps this method is too vegetable, but please do not be laughed at. ：）
Examle:
<a href= "Add.asp?action=add" >
<a href= "Add.asp?action=delete" >
The handler functions are as follows:
Action1=trim (Request.QueryString ())
If left (action1,7) <> "action=" Then ' qualify querystring must be action=
Error (ERR01) ' Fault handling
Else
Action=request.querystring ("action") ' Gets the value of querystring
End If
Select Case Action ' handles the QueryString
Case "Add"
.....
Case "Delete"
......
Case Else ' If QueryString does not have this value then error handling
Error (ERR02)
End Select