The topic of physical layer key agreement in Gaussian block fading channels in the presence of active adversaries is investigated in the present contribution. Frequency hopping versus frequency spreading are studied as potential defense strategies against denial of service (DoS) attacks in the form of jamming. We begin by investigating the optimal power allocation policy for the jammer -- in the case of frequency spreading -- when side information is available. Through numerical evaluations it is shown that effect of the availability of side information on the jammer's impact is minimal. Next, the competitive interaction between the legitimate nodes and the jammer is formulated as a one-shot zero-sum game; it is found that under short term power constraints, the legitimate nodes should not always employ frequency hopping while the jammer should always use frequency spreading. This result contradicts the common practice of using frequency hopping to defend against jamming attacks in standard wireless systems.