As @nicklear mentions above, one option is to replace the lockscreen in its entirety. (a workaround based on ‘there’s an app for that’)
I’ve not tried any personally, and I’m hesitant to recommend any specific one. I’d recommend searching for ‘lockscreen’ in your favourite app store to check what the options are. There are quite some review sites out there that can help you decide as well. Some lockscreen apps will have specific features that you might find useful, such as seeing notifications on the lockscreen (see a discussion about that on this forum can be found here).
As always, your mileage may vary. Also note that if you use a custom lockscreen to set non-standard unlocks (e.g. a 5x5 pattern grid), you can be locked out of your phone if the app stops working for some reason (app/system update, gremlins). Having a backup before experimenting with any apps that are designed to keep people out of your phone is always a good idea, just in case the app decides you’re one of the people it wants to keep out.