NO, they can not. If you have taken possession and they are denying you access, they should be charged a "fine" for every day after the possession date set at closing. If they blatantly refuse, call the police. It is your house now, not theirs.
I want to ensure I properly understand your question. Have you closed on the property yet? If not, as another agent has indicated, you should be allowed a pre-closing walk through. Has the seller indicated why they are denying you access?
Century 21 Diversified
Did purchase agreement give you possession at closing. If yes than the seller has breached the contract and there could even be a penalty in the contract on a day by day basis. What does your REALTOR say? The reality is the only remedy is to go to court and the resolution will not be timely or cost effective. This is just one of the reasons a Great Realtor would make sure you had possession at closing with a pre closing final walk through to make sure everything was out and in good condition.