You can reset your account which will allow you to change your avatar, name, and essentially start over (you'll go back to 0 stats and an empty dorm). You will still keep your awards, friends, etc. Resetting will not remove any silences, permanent or temporary. There is no "guest" mode; your account is linked to your device's unique identifier. Having more than one account on one device is against ToU and could result in a ban.