You want to block the entire hotel due to a wedding from 1 july to 3 july.
1) Restrict checkins and stays, for all roomtypes on the hotel, for the period, under "Settings -> Restrictions -> Restrictions for check-in, check-out and/or stays". We give the restriction the name "Wedding"
2) Go to "Settings > Price Changes > Promotion & Discount Codes > Create a new code." In the field named "Override restriction", you'll find the restriction "Wedding" which you just created in step 1. Choose that one. Save you new code.
Provide the guests with the code, and information that they can click where it says "Do you have a promotion code?", and they will override the restriction.