This is an old troll move.
There are many things you can do to avoid it.
1) Obviously, avoid Open at CGs.
2) Block suspicious CMDRs (anyone flying a small shieldless ship badly).
3) Stay below 99m/s (100m/s is too fast), if flying a smaller ship, stay below 90m/s to avoid being rammed over the speed limit.
4) Revenge. Grab any cheap ship, and basically chase them around, crashing in to them (below the speed limit). They'll soon get tired of it. I spent a while in my Python with the gear down (90m/s limit) just flattening one guy as soon as they undocked. Hilarious.

5) Bait. Be the bait, fly a larger ship, but one that can stop easily, slightly fast. When you see them charge at you, drop below the speed limit. They go boom, you do not. Repeat until they ignore you.
6) Create your own KoS (Kill on Sight) list. Add them to it. If you see them ever again, kill them. Especially rewarding if you can kill them with a troll move.
CMDR Cosmic Spacehead