My suggestions:
Backup your data (export with Data Loader)
Convert field from Picklist (Multi-select) to Picklist. It will probably clear all the values in the existing records from that field. If you check the option to not strictly enforce picklist values - data loader should allow you to upload previous values and you'll see field values like "Value1; Value2", even though it won't be valid value.
Consider how to cleanup records - either manually or with some import process
Of course I'd recommend trying it out on full copy sandbox first, then in production. Also check if no code / workflows are based on the existing field (because the system won't allow you to change it so easily)
Another option is to leave current field, hide it and create a new one, but that is an easy approach that is just keeping mess in the system.