Jon,
You are correct, hand weeding is the obvious answer, but the other obvious answer is applicable to growing any turfgrass in efforts to keep out undesirable species: go back to the fundamentals of turf management. Specifically, find out EXACTLY what needs your desirable species has, and what makes it different from the undesirable species and adjust your maintenance practices to HEAVILY favor the desirable. Sounds easy enough, but often is very hard to be so exact and precise in practice.
For example, in this conversation we want to promote bentgrass and keep out Poa. I go first to my golden rule of turfgrass management: The best defense against any pest is a dense, healthy stand of turfgrass. So there's your first answer, you need to do everything it takes to keep the bentgrass dense and healthy ALL the time. There are two other management areas that deserve specific attention in this situation.
First, irrigation and fertility. If you do your research, you will find EXACTLY what levels of fertility and soil situation bentgrass enjoys. Look at how this differs from Poa. Specifically, bent prefers lower inputs of nitrogen than Poa. It also prefers more acidic soils than Poa. And as a more minor aspect, it tends to benefit more from iron applications than Poa. Then look at the ways you irrigate. Bentgrass naturally wants to be deep rooting. Poa is a clump grass with shallow roots. Deep, infrequent watering will encourage the bent to go deep while drying out the shallow roots of the Poa. Essentially, with just these two elements, you can slowly but surely "starve" exisiting Poa as well as new seedlings trying to emerge all the while pushing the bentgrass to be more healthy and vigorous and ultimately outcompete the Poa.
Secondly, you have to think hard about cultivation, including aerification, verticutting, spiking, scarifying......basically anything that damages or opens up your bent to invasion. Poa simply cannot take foothold in a 100% dense stand of bentgrass....it can't find the soil and if it does, it can't find sunlight. But no stands are 100% dense. Poa comes in when and where opportunity presents itself in the form of a thin or open area of turf. Some of these you have a hard time controlling, like ball marks. But all of the cultivation practices you do have control over. Some courses choose not to aerify. I'm not saying that. Bentgrass needs and likes cultivation, but it's a matter of choosing the right timing, priming to the bent to recover as quickly as possible with shots of fertility, and to take all possible measures to prevent Poa from taking hold in any wounds you do inflict.
There are innumerable ways to go about doing this, and I don't even claim to know half of them, but keeping the fundamental ideas and understanding about the differences in the two grasses and making a dedicated, concerted effort to create and maintain an environment more conducive to the success of one over the other should help you obtain the results that you are looking for.
And in regards to the second part of your post.....
The answer goes hand in hand with what we're talking about above. The benefit to a "monostand" (though I really don't think that's a great description if you're looking at a literal definition) or one grass SPECIES type of green, is that you only have to worry about maintaining ONE grass! As we just talked about above, different species, while similar, still have stark differences in the way they prefer and OUGHT to be maintained. With one grass species, you don't have to play around trying to find the right mix that will keep both grass types happy. For one specific example, look again at irrigation. If you have a 50/50 blend of Poa/Bent, you have a shallow rooter and a deep rooter. Depending on climate and time of year, the bent on a USGA green could probably go with 1-2 deep waterings a week and be 100% fine. The Poa on the same green would be struggling, unless supplemented with some serious attention to handwatering or light daily syringing, which is what many supers in this situation do. I enjoy the fact that with mostly pure bentgrass greens, I don't worry about the Poa drying out because there's not enough of it make a difference, and I water max 3 times per week in the high heat of the summer and only had to handwatering the interior greens mounds twice all year. With more Poa than I have, I'd never get away with that.