You can't do that without it trying to remove all that stuff.
daemo...@studypad:~$ sudo apt-get remove mozilla-plugin-gnash
[sudo] password for daemonax:
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer
required:
libecryptfs0 libparted0 ecryptfs-utils cryptsetup python-pyicu keyutils
Use 'apt-get autoremove' to remove them.
The following packages will be REMOVED
mozilla-plugin-gnash trisquel-desktop-common-recommended
trisquel-gnome-base-recommended trisquel-recommended
0 upgraded, 0 newly installed, 4 to remove and 2 not upgraded.
After this operation, 250kB disk space will be freed.
Do you want to continue [Y/n]?
This is a big pain, because even if Gnash is being actively developed it is
still not very good, and doesn't work with many of the websites that I used
for watching videos on like http://www.youku.com which worked fine with
swfdec.
I think it would be best if neither are included by default, or at least are
not tied up with the system in such a crucial fashion.