This simple excuse works perfectly if you are a business man or a doctor. You could excuse yourself to go to the bathroom and then say that your office called and you are needed there in any case. The other person will get a hint that you want it to end. This will ensure that you don’t hurt his feelings and say goodbye on a very pleasant note.
A family emergency can be a quick escape. You need to be a genuine actor to pull of this one. Show that you are tense and very worried. Make it sound urgent and very important. The excuse could blow up if the other person is very courteous and asks you if you need a ride till there or is there any sort of help needed. You could show that you are big enough to handle it and want to do it alone. Tell them that you will call them back after this is over.
If you are really desperate to escape, spill something on your dress. Make it seem very natural. You could either spill the drink or the chocolate sauce on you dress and then get panicked. Tell the other person that you really need to change and that you will a date for some other time.
Tell the truth. This might be a difficult thing to do but will end the agony once and for all. Build up some courage and say it that you are not enjoying this and this should end. Say goodbye on a good note and tell them that you should be just friends.