This is because one feature of JComments which stores last used name and email in cookies and restore these values when user (guest) opens the page with JComments form. You can remove this behavior by small modification:
1. Open file /components/com_jcomments/js/jcomments-v2.3.js
2. Replace code:
storeValues: function(){for(var i=0;i<this.store.length; i++){try{var el=JComments.prototype.$(this.id+'-'+this.store[i]);if(el){JComments.prototype.setCookie(this.store[i],encodeURIComponent(el.value),14);}}catch(e){}}},
restoreValues: function(){for(var i=0;i<this.store.length; i++){try{var el=JComments.prototype.$(this.id+'-'+this.store[i]);if(el){if(el.type&&el.type=='hidden'){return;}else{el.value=decodeURIComponent(JComments.prototype.getCookie(this.store[i]));}}}catch(e){}}},
with
storeValues: function(){},
restoreValues: function(){},