I want to corrupt a datablock(!)

[chunyuh]

Oracle itself provides a tool called bbed.
Normally Oracle support use it for repairing blocks. Of course, you can use to corrupt blocks.

You need to compile bbed urself.
$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk $ORACLE_HOME/rdbms/lib/bbed

Thanks