there is interesting requirement came when people started testing on some of the OOTB application, we usually develop rapid OOTB application in SharePoint 2010. One of our tester is started testing the application, he was deleting the items from the look up column and customer wanted to retain those values it the list so that they can identify what was the value previously
this can be achieved in two way
1) if you are designing the form in in InfoPath, instead of look up column, create a single line text,
2) make that as drop down
3) then bring the data from the other list and on the form load bind the data.
4) so item stored the date in a single value but user can select the data from the looked up list
1) removed the form field from where you are looking the data from the newform.aspx
2) create SPDROPDOWN control rename the IDs and all the data as per the FORM FIELDS
3) create a connection for the look up table ( you can filter also in connection like active lookup values)
4) add this connection data to SPDROPDOWN
5) save the file. it works as expected
Sorry I am not putting any snapshot. if anybody wants i can put those things.
thank you. happy coding OOTB