1) Increase the rate to something truly insane like 1000%
2) Introduce currency controls and force everyone to use Ruble
Neither of these options are good, but #2 seems much more likely.
I don't know if it's going to be effective at all, just trying to find the most charitable explanation for CB.ru actions.
Where it does make a difference is that long end bonds (OFZ 28s) are now yielding 15%. That's 15% for 13 years ;) Chunky yield in the long end. That will tempt the long term investors in (IF big if, they don't impose controls). Care to catch the falling knife?
It will be a bit more complicated this time.
They might not have any other option though.
The more relevant short term argument that I am referring to, is that if they were to "squeeze" the offshore market by disallowing or limiting the lending of RUB by local banks to offshore banks, then they could easily engineer a massive squeeze on the rouble offshore. That is all those people who are short RUB would suddenly find that "rolling" those positions (that is, borrowing RUB so you can sell it), suddenly costs a fortune. This is even true in FX forwards because ultimately these are a derivative of interest rates. If it costs you a fortune to "fund" the RUB, then the converse is that it pays you a fortune to lend RUB to the market. If it pays you handsomely simply to lend short-term RUB to the market, there is no reason to buy long term bonds, whose price must "compete" with the front end of the yield curve. In short: high short end rates will deter purchasing of any instrument with a (significantly) lower yield in the long end of the yield curve. If we were to get the above mentioned RUB squeeze to anything like 100% or higher, imagine how silly it would be to buy a long end bond at 15% (even though yes yes, long term it might still be a good investment - but that would require a long discussion about the implied path of future rates. Humans have proven themselves to be very short termist in markets and thus it is likely that long term bond prices would fall, dramatically in that scenario).
So to answer your question directly, bond investors still would be able to trade out of their positions (though in a disorderly market even that cannot be taken for granted), but at a very low price.