I just went through this yesterday while replacing the discs on my w203. Took a lot of pounding and liberal use of penetrating degreaser. My buddy and I had to take turns wacking away at the thing. We used a standard hammer, so it probably would've been easier with a heavier hammer. Glad to hear I'm not the only one dealing with fused discs/hubs. The wheels themselves can also fuse to the discs.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)