Implementing a strong password policy that requires complex passwords, which typically include a mix of uppercase letters, lowercase letters, numbers, and special characters, can greatly reduce the effectiveness of a dictionary attack. These attacks rely on trying common words and simple password variations, so complexity directly counters this method. The other options—increasing the frequency of backups, installing antivirus software, and limiting login times—may improve security in other areas but do not directly address the vulnerability exploited by a dictionary attack.