if they know the values, you can have them enter a comma delimited string and then you can check if the value is in the string, or you can break the string into an array and check if it is in the array, or you can pass it to your stored proc and have a helper function that will split it and return a table you can join against.
I am sure that there are a couple more ways to get it done, but those are the ones that come to mind.
HTH