In ad hoc wireless networks, end-to-end TCP is well-known that it can obtain very low throughput with increase of the number of hops. One promising way to resolve this technical problem of TCP throughput degradation is proxy-based TCP. In proxy-based TCP, an end-to-end TCP session is divided by (an) intermediate proxy host(s), which leads to reduction of hop-count of each divided session and improvement of TCP throughput. However, we show that proxy-based TCP surprisingly does not bring performance improvement and obtain lower throughput than conventional end-to-end TCP in a realistic situation of limited buffer space in proxy host(s). The reason for this degradation is interruption of segment transmission caused by wireless channel capture around the proxy host. In this paper, for resolving this problem, we propose intentional window flow control which prevents wireless channel capture around the proxy host. In our proposed intentional window flow control method, the proxy host and the destination host return advertised window size which is intentionally set to small value almost independently from its vacant buffer space. Performance evaluation results show that our proposed intentional flow control with small advertised window size prevents wireless channel capture around the proxy host and significantly improves proxy-based TCP performance.