I've gotten this question a couple of times already, so for all of you, here's how you can make a Toy Box playable by only one character:
When submitting a Toy Box to Disney, on the bottom left of the screen, there's a section called "Add Filters". In it, there's four categories:
1- Primary Genre
2- Secondary Genre
3- Character Required
4- Abilities Required
The last two are the ones used to restrict the level to certain characters. Character Required restricts it to only one particular character, while Abilities Required limits them to a certain subset of characters. After selecting the character or abilities, accept the changes (using Square on PS3/PS4, X on 360/Xbox One) to close the filters menu and then submit your Toy Box to Disney.
If you have any more questions regarding the subject, let me know!
The opinions posted here are my own, and not my employer's.