The first thing I'd say is that if the only problem is your city boundary expanding over the lair, you can put off going up a population level indefinitely by building pioneers, casting Sacrifice, or whatever other options you have to reduce population, and then you just have to remember not to build any zone expanding buildings (monument and town hall are the main ones).
Re: the general case, it can be scary and inconvenient having stacks you can't kill wandering in your borders, but bearing in mind that they generally move one tile randomly, in my experience they don't normally do too much damage. Just don't stop your stacks next to them. Note however this will change if the AI is changed so that monster stacks start targeting cities (as opposed to moving randomly), but I haven't seen this. They did seem to be targeting my stacks more in the last 1.2 game I played, but I didn't really have a chance to observe any large stacks near cities.
I don't mind particulary if they target stacks more because you can generally move out of their way using roads etc., but if they start targeting cities from > 1 tile it would make some random events very hard to recover from. Even then though, if they can only see two tiles, then a random walk from your border would make them quite unlikely to kill more than one city. But I personally hope wandering monsters don't start targeting cities from more than one tile away.
