If you want to experience a tech victory, you probably should disable cultural victory when you start a new game. It's in a box on the lower-right corner of the screen, below where you choose the galaxy size. You are playing with version 1.03, right? There isn't any way to change this with a game you've already started.
If you want to prevent a culture victory, go to the most recent autosave and declare war. Or, give culture starbases to the AIs. Or, give planets to the AIs to decrease your influence. You have your game autosave every 12 turns, right?
If you want to play until the bitter end, you can disable all 3 victory conditions (cultural, tech, and alliance). The game will last until every other major civ is eliminated. Don't do this on a gigantic map, as you may not have the patience to finish the game.
However, continuing after winning isn't really that interesting. After experiencing each winning condition once, you probably should leave all 3 victory conditions enabled. After a few games, it isn't fun to continue once you're dominating. The game is specifically designed to end once you are in control.
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