You can use regular expressions to add constraints. It won't prevent them from inputting the characters outright, but it will fail validation, then you could add a custom error message that tells the user only numbers and letters are allowed.

The following regex requires one or more letters or numbers, but you can look up more about regular expressions if you want to require more than one or anything else.

[a-zA-Z0-9]+

The default validation error will just say "invalid input" so I'd highly recommend adding a custom one for that field that says something more specific like "Letters and Numbers only"

Depending on which version of the Form designer you're using, it's possible to further restrict things with JavaScript, but regex is the easiest to implement and maintain.