No, of course the FA can't stop United letting PK onto their private property. What they can do, however, is have a rule than any club participating in their competitions must adhere to their anti-drugs regime which includes ensuring that sentences passed down by the relevant Tribunal are enforced.
Thus, if PK is banned from traning on United's property and United let him train on the property, they would face sanctions from the FA (fines, points deductions, banning from competitions and so on and so forth). Obviously, faced with such potential sanctions, the club are not going to let PK on their property to train (assuming he is so banned), whatever their theoretical right to do so.