Hi God, first, as you say, covering a town with one station using non-connected bus stops IS abuse and is already not allowed under the rules. If you see this ask the player to stop and/or use the !admin trigger.
Secondly, about the amount of bus stops in a town. This is not as clear cut as you suggest, bus stops are often used to grow a town in order to accept goods (at least in temperate) 5 bus stops with good ratings will grow a town very fast. Using bus stops in this way will benefit any company using the town as it will be growing fast, producing more passengers/mail.
Having so many stops that you deliberately block up a town is already against the rules as they stand (no blocking) - but having a set number of stops allowed per town would prove frustrating to most players.