When we replaced our cloth we used a couple techniques
Before anything we ripped off the main portion of the cloth so only the portion left in the channel was there with maybe a half inch sticking out.
Then we used a punch/hammer or screwdriver to get a small section out.
From there we used vice grips or channel-locks to grip it, then twist using the hub as a fulcrum point to pull it out.
Some of the cloth (depending on what type it is) might have a plastic core in the channel section, if you can get a grip on that and pull the core out it makes taking the rest of it out much easier. Also, using a heat gun to blast the section in the channel also makes pulling it out easier.
Its not an easy task, but it is attainable, although if you are at the point that its that hard to take it out the channels are probably warped or has a corrosion build up under the cloth and you're probably better of replacing at least those bottoms hubs as well.
Just note this: It kinda sucks doing this. Back when I was an assistant manager we closed down for a morning during the summer to give it a go and my old store manager ended up passing out from a combination of frustration, heat, and dehydration (granted, its not summer now, but it just shows how difficult the process can be).
Just another side note, taking the hubs off may make it easier to get the cloth out. We did that for one of them and used our bench-vice to hold the hub while we pulled the cloth out.
Hopefully my rambling makes sense and good luck!