Politely tell them that you would rather they did not come into your store any more and that if they return in the future they will be trespassing. You can then order them to leave.
It is even better if you can write to the person concerned, assuming you can obtain their name and address.
Send the letter from yourself or, preferably, ask your solicitor to send a letter to the person concerned.
Say that the person is no longer welcome in your store. Give a warning that should the person return to your store in the future, he or she will be trespassing.
Send the letter by recorded delivery and keep a copy for yourself.
By writing to the person, it avoids confrontation and you have written proof of your actions to produce in the event of any future problems.