Even i have experianced the same.if continous ping has given it had stopped with --->cntl+shft+6 (all together) then x ,
but in the case of traceroute .....many times we have to wait till the 30th hop
For me it works always.. Actually you dont even need to press x after the combined sequence
Type escape sequence to abort.
Tracing the route to 184.108.40.206
x is not necessary, sometimes even not good to use (while connected to from one router another router via a Telnet session and running traceroute there, it would just return to the first router session).
pressed several times might help.
In my experience the X is not needed and as Milan points out is sometimes disruptive. I find that frequently I must repeat the key stroke sequence several times to stop a traceroute, but I find that the keystroke sequence of CTRL + shift + 6 is effective in stopping traceroute (or extended ping).