Is the supplicant's saying, "O Lord, if this is the only prayer I will have answered, then make it that I never return to disobeying You," forbidden, and does it mean that his prayers will not be answered in the future?

It is not forbidden to supplicate to Allah to abandon a sin; rather, it indicates an earnest desire to obey Allah. This does not mean that subsequent supplications will not be accepted, for Allah Almighty answers every supplicant who calls upon Him with sincerity and devotion, as stated in the Noble Verses: "And your Lord said, 'Call upon Me; I will respond to you,'" and "And when My servants ask you concerning Me, then [answer them], 'Indeed, I am near. I respond to the invocation of the supplicant when he invokes Me.'" So strive to supplicate to Allah and ask Him for whatever good you desire in this world and the Hereafter.