while mining and there are fragments floating about, press 1 to open the left hand control panel
go to contacts
there will be a list of all the fragments out there
on the list of fragments move down to one that has a metal that you don't want to collect, (if there are two different in a single fragment it's okay, you will get a choice between the two)
once on the fragment that has the metal you don't want to collect, press space, it will then give you the option of ignoring it, move to yes and press space
same applies to un-ignoring something, open contact panel, find a fragment with the metal you no longer want to ignore, select it, choose do not ignore, press space
there is no actual ignore list you can view unfortunately. an actual list would be great that you can check and uncheck at any time...like back in the hangar at a base
however until they implement that (let's hope they do) this is the way you have to do it.
note: my understanding is that the ships computer remembers what you want to ignore, so in that sense it does maintain a list, just not one you can access. I have yet to confirm this myself, that the computer remembers the list, but several people have said that it is remembered
this guide:
https://www.reddit.com/r/EliteDangerous/comments/4wmgja/general_mining_guide_20/
has a section titled "What to mine" and gives prices etc....but from that list you can see that there are only a few that are considered trash. But I like that it breaks it down into top 3 or 5 as well as commonly requested for missions etc.
this guide:
http://elite-dangerous.wikia.com/wiki/Miner
under availability and value, shows minerals, metals and chemicals, with rough values, but most importantly what types of rings you can pick up the item at.
please note that I am very new to the game and mining, and all the info above I have gained from research on various websites, youtube videos and here from the forums. very little of it is from actual personal experience. At this point I have only mined once, but this did show me how to use the ignore feature as well as how to un-ignore.