I don't think there is any reason to make them change it unless Google complains (unlikely; I am guessing Tumblr is much more visible than ispyapp.com) - I coul

I don't think there is any reason to make them change it unless Google complains (unlikely; I am guessing Tumblr is much more visible than ispyapp.com) - I couldn't find any ToS for signup about appropriate usernames. You might want to inform the user that using trademarked names is frowned upon and offer to change their username, that if Google threatened your site over it, you might have no choice but to change it, and you don't want to make a dick move and just change or delete the account