Edit: Sorry misread the post, this I guess would be for others.
To get rid of missions Press L (default) will bring up the communication log, then the far left tab (forgot name) is the mission tracker. Select the mission in the left hand column and on the bottom right is the action to drop the mission. You can also make a mission primary so it will be on top of your tracker.
Also to note is the share button, this allows you to share the mission with your team if they have already completed said mission (at a reduce xp rate and no items reward).