Look at the pickup pattern, and imagine that shape in 3D space around the mic. Scale appropriately to make sense to you, so that what's inside that 3D shape is what you hear. Now move that shape around, by moving the mic or changing the pattern, so that it picks up what you want and not what you don't. You might need additional cabling, to have it in a different location, and/or a different mic with a different pattern, or whatever, but that's how you do it.
Unfortunately, lack of budget does not change physics. But likewise, an overabundant budget does not change physics either. Everyone has the same constraints there.
It would probably help too, to have multiple mics in different locations, and mix them all together. Then even if you do get a loud conversation that just happens to sit next to one of them, the others can still drown it out somewhat. But of course you should also try to not have it close to any specific seats in the first place so that it just doesn't happen. Likewise for a super-directional pickup pattern that "reaches out" to some specific seats, even if they're somewhat far away.