Account lockout mechanisms are an effective mitigation strategy against brute force attacks because they limit the number of failed login attempts before temporarily disabling the account. This prevents attackers from trying an excessive number of password combinations in a short amount of time, thereby thwarting brute force attacks.

Implementing stronger password complexity requirements, while beneficial to overall security, does not directly mitigate the risk of a brute force attack since attackers can still attempt numerous combinations within the complex requirements. Intrusion detection systems, encryption, and VPNs are security measures that protect against other types short of attacks and do not specifically address the issue of preventing password guessing. Challenge-response questions add an additional authentication step but can also be susceptible to brute force if not properly limited.