I have a heavy list of products that have a special price until the end of March, unfortunately, as I wasn't told how long the discount will last, I never set an end date.
I'm not overly proficient in SQL or PHP, so I wouldn't know how to approach SQL query that would set the end date to March 31st or simply remove all special prices.
You can do that from the backend. Just go to Catalog->Products check the checkbox near each product you with a special price and select from the top right corner the action 'Update attributes'. You will see a screen with all the attributes that can be changed. Set 31st of March as the 'Special price to date' and save.