How can I somehow block Color Codes? I tried to send the message always in §e(yellow) but color codes are still working.
It depends, if you want a certain player to NOT SEE COLORS, use the method suggested by @shoghicp, but if you want to disallow players to SEND COLORED messages, then use the one that @EvolSoft told
Also players can't send colored messages, they need to be explicitly allowed by a plugin to do so (check remove format on Player)
That is wrong. The methods have changed for 1.5 setRemoveFormat() to true will disable players SENDING colors, and it defaults to true. You have to set this to false so players can talk/put signs with colors directly.