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make the frame unusable
Isn't a big crack in the seattube enough?
Seriously though, I guess that chopping the seatstays is probably the safest place to disable a frame and make it look like you've really trashed it.... Then you can get it all welded up nicely
e.g. from this
to this
Option 2 is to photoshop a chopped frame (e.g. change the colours on my photo above) and see if you get away with it
Also bear in mind that sharing dastardly schemes on the worldwide web may also affect your warranty and I'd hate to encourage dishonesty - even against a great corporate swallower/diluter/destroyer of legendary brands (bontrager, lemond, fisher)
I think stop being a chancer.
It seems that was your 666th post, no good can come of this.