By default, accounts are restricted from accessing S3 unless they have been given access via policy. However, S3 is designed by default to allow any IP address access. So to block IP's you would have to specify denies explicitly in the policy instead of allows.

Allow Access to Specific IP Addresses
 

 <div class="code">  
 {  
   "Id": "S3PolicyId1",  
   "Statement": [  
     {  
       "Sid": "IPDeny",  
       "Effect": "Deny",  
       "Principal": {  
         "AWS": "*"  
       },  
       "Action": "s3:*",  
       "Resource": "arn:aws:s3:::bucket/*",  
       "Condition": {  
         "IpAddress": {  
           "aws:SourceIp": "54.240.143.188/32"  
         }  
       }  
     }  
   ]  
 }  
 </div>  

Restrict Access to Specific IP Addresses
 

 {  
  "Version": "2012-10-17",  
  "Id": "S3PolicyId1",  
  "Statement": [  
   {  
    "Sid": "IPAllow",  
    "Effect": "Allow",  
    "Principal": "*",  
    "Action": "s3:*",  
    "Resource": "arn:aws:s3:::bucket/*",  
    "Condition": {  
      "NotIpAddress": {"aws:SourceIp": "54.240.143.188/32"}   
    }   
   }   
  ]  
 }