Why don't you resign instead?
It's a question that looks like it has an obvious answer: because I'm winning! But that begs the question "who cares?"
Let the game end. What happens? Your record goes up by 1 win, and you get some rating points. In the end, other than the sheer enjoyment of the game, that's the only real consequence. A few bits in a computer tracking your record.
Now if you were merely bored with the game, and convinced it is utterly over, nothing stops you from just resigning a winning position and going on with your life. Perhaps you might play another game in the saved time.
Which means, when you really get down to the tradeoff, you are pairing the points you might lose by resigning yourself (to solve your interest issues) against any sheer enjoyment of the game the other party may have.
Which leads to the human question: do you have a sense of how much enjoyment they have left in the game? Clearly they're not already at the point of saying the game was no longer worth playing, or they would have resigned already. So you are asking them to truncate that enjoyment for your sake. You are really telling them that they are no longer worth your time and they are not worth the points it'd take to break free of the game of your own accord. Whether they agree is really a people problem, and you may get some clues from their move choices and/or their chat.
Maybe they've been looking to try out that difficult endgame to see what they might be able to pull off. They'll never find out if they resign. But now they know that any fun they might have trying out that end game is paining you.
There's a few alternatives I might suggest. One is to do what Allure did in the comments:
Black is completely busted. 39. Kxe6 Kg8 40. Ke7 and the pawn promotes, or 39...Ke8 40. f7+ Kf8 41. Bxc5+ and 42. f8=Q+
Rather than telling them to resign, because they are no longer worth your time, help them see what you see. Practice teaching people how to analyze the endings. Then they can decide whether that means its time to resign or if they want to play out a line you didn't analyze.
Alternatively, realize that those bytes in some computer's memory banks somewhere are not really all that important. Make your own game. If a position seems totally won, look for a challenge instead. See what non-optimal play you might be able to bring into the game to make the endgame interesting again. Maybe you need to hang a bishop, or promote a pawn to a funny piece. If you have a text-to-speech engine that can call out moves, consider playing the ending blindfolded, just for the challenge. We teach games to be all about winning and losing, but at the heart of it they're games. Find ways to have fun with them!