You may have had a Crush with another user and suddenly change your mind - which is why you can cancel any Crush!
If you have already chatted with this Crush, you can go to the “Messages” tab and swipe left the conversation you had with this Crush. You will then be able to select the option “Cancel this Crush”. You can also open the conversation with this Crush, click on the three dots in the top right-end corner of the conversation and then select "Cancel this Crush". Once you have cancelled a Crush, don't worry, the person won't have access to your conversation anymore and will see when opening it that the Crush is unavailable.