How to stop pasting pre-made answers?
Ensure that a player cannot see the same puzzle twice (regardless of the mode).
- it probably cannot fix the leaderboard retroactively (not a problem in the long term)
- it will lead to a lot of players being unable to clash until the puzzle pool grows
- it can be bypassed with multiple accounts (but will people really create multiple accounts for that?)
Another option is to take into account the repetition of the puzzle in the ranking. For example say that only the first clash with a given puzzle (no matter the mode) counts in the ranking. This way people can keep clashing at will, but they will have no intensive for cheating with pasting. The first issue may be still valid (if the logs don't contain info about the puzzles done), the second is not, the third is.
Disabling pasting in the IDE is a terrible idea which will hurt genuine players (they may be using an IDE to write code) and leave genuine cheaters in peace (they may write a script to autotype their code in the IDE window).
With regards to points: you can get around 13k points from achievements, 3k from contests, and 1k for each optimization/multiplayer game (10k at the moment). 5k for clashes sounds rather reasonable. Keep in mind that as the pool of problems grows bigger, looking for the correct code to paste will become longer than actually typing it in.