Search your opponents frontiers for streets that are next to your frontier. If you have found such a street, place a flag directly next to the street, click on the flag and click the symbol for connecting the flag to the street (that green symbol with the red arrows). If it didn't work, the street wasn't near enough.
You see, this way you could then delete the flag and cut the street. But DON'T. There are much better things to do.
First, place a storage hall next to the flag, and connect it with that flag only, but not with your own streets. Soon you'll see the opponnent's settlers building the hall on your land for you. You can do even more. Let them build anything for you. This is even more interesting if it happens next to a smith, because then the weapons are being transported into your storage hall, where your knights grow (with the opponent's weapons, hehe).
But don't forget: Never connect this new street system to your own system before disconnecting from the opponents, because otherwise your resources, people and knights might go into your opponent's land and help him instead of you.
If you are not satisfied with the random generated maps of the settlers and want to modify some maps, you should try EdSet08.lha from AmiNet/game/patch/. The kewlest thing is the Mega-Geologist.
On the mission select screen type "PASSIVE" and you can play any of the 30 levels.
01---"START" | 11-"CHOPPER" | 21--"PASTURE"
02-"STATION" | 12----"GATE" | 22----"OMNUS"
03---"UNITY" | 13--"ISLAND" | 23--"TRIBUTE"
04----"WAVE" | 14--"LEGION" | 24-"FOUNTAIN"
05--"EXPORT" | 15---"PIECE" | 25----"CHUDE"
06--"OPTION" | 16---"RIVAL" | 26--"TRAILER"
07--"RECORD" | 17--"SAVAGE" | 27---"CANYON"
08---"SCALE" | 18---"XAVER" | 28--"REPRESS"
09----"SIGN" | 19---"BLADE" | 29-----"YOKI"
10---"ACRON" | 20--"BEACON" | 30--"PASSIVE"