This may sound like a stupid idea to you, but as you are still wasting sperm regularly to this tempting person, I will just commiserate with you and say what works for me and some other people I know. Pray for hert (or him, or whoever this 'ploni' is). Daven for them right as you are going to sleep from the bottom of your heart, no matter who it is. Ask Hashem to help this person with health, wisdom, saving from making the kind of mistakes that so many of us humans make somethimes like way misguided life priorities, etc. Beg Hashem to give this person a true relationship with Hashem - even a giy needs to be aware of the thruth, that there is a G-d and that G-d created him or her for a real, significant purpose, whatever it may be. Choviv Odom...all odom, and chavivin Yisroel is beyond that. But we all desperately need these prayers you will utter on your bed (if you choose to) - from the pritzus goyishe schmutz model, to the beautiful gadol baTorah who is a Kadosh. We all need these same things, each at his or her own level. Gotta start somewhere. Let your prayer be that start for her, him, whoever it is.
And include yourself by name at the end and ask Hashem to give the same thing to you.
BTW, Rav Yaakov Koppel, zy"a, a very great 17th century mekubal has a tfilah that is very similar to this idea. And there are gemoras in Brachos that seem to indicate the same thing, too.
Go for it. Drop the fear of the person - for your fear of 'ploni' is what gives 'ploni' the power he or she has. Forget about trying with all your might to distract yourself from "thinking of 'ploni' c"v". You already are, amigo. It's not working. And instead, do something positive for this person and for yourself in turn. For 'ploni' is an eved Hashem, too, as you and I are, chaver. As we just said a few days ago in our davening: "Ki hakol avodecho"!
Just the humility of letting go of your old ideas and dear old ways may be the 'zechus' (or rather, 'zakus') you need to have Hashem's help keeping you clean tonight. Leave the challenge in His domain, totally. Have no fear at all. As Yesod Yosef writes, the fear of having a wet dream is one of the greatest causes of it - "v'es meguroseihem avee aleihem", the novi wrote. This prayer thing is a new way for you. Please consider it. And stop asking Him to protect you from the keri, for Heaven's sake. It just triggers fear in you, I bet.
Hatzlocha with trying something totally new, and I wish you patience, as no derech is foolproof. Try it and see how it works for you.
- Dov