This paper investigates the optimum power allocation ratio of a jammer in order to ruin the sum spectral efficiency (SSE) of a user-centric cell-free network. In this regard, signal to interference plus noise ratio (SINR) is initially derived for each user in uplink, and then, a closed-form expression for SSE is provided. An optimization problem has been solved for two scenarios which finds the optimum power allocation that minimizes the SSE of the network, first for a single antenna jammer and next for a multi-antenna one. Additionally, the performance of the SSE has been studied in the presence of several single-antenna jammers in the proposed network and also one multi-antenna jammer. The result is compared with co-located multi-input multi-output (MIMO) network and it is found that unlike co-located MIMO, as the number of serving access points (APs) increases, the network becomes more robust against jamming. The effect of the jammer’s antenna on power allocation ratio and spectral efficiency is further presented. Moreover, the impact of the number of jammers and the number of serving APs on the SE has been surveyed.