I know when I buy a new car or new house I want to take EVERYONE for a ride and compel them to celebrate with me. Sometimes the excitement overruns self control.
So, just say, NO.
The buyer will be entitled to access the home they are buying during the home inspection, and often during the appraisal. Then the final walk through.
Now, what if there was no signed purchase agreement? What if they were still interested buyers? Would you ban them after three visits?
At some point a buyer may want wise Uncle Mo and the in-law providing the down payment to give their blessing to the purchase. Then comes the, "Will that 70 inch flat screen fit in the fireplace and will all the pieces of the sectional fit in the living room?"
If you say NO to soon you just might kill the golden goose. It is to your advantage to keep the excitement level as high as possible. When the level comes down, what replaces it is buyers remorse.
Advise your agent you would like to accommodate the buyers needs, within reason, defined by, 2 more visits and no more. THEN be flexible.
After three visits......the next one should be the "final walk through" just prior to closing.
I haven't had any trouble with this scenario if I educate my buyers beforehand (like a good agent should).
If it bothers you to have them constantly coming over (and they should always, ALWAYS have their agent with them), just tell your Realtor and limit the visits.
Good luck to you!