What's the best way to deal with a database with a big table with information that will RARELY be used? I think useless is a heavy word for this.

We have a database with 500GB, and most of it's information is only there if we MAYBE need to use them. it's a big database with credit card information and until now, we've never had the need to select them ( the table has 500millions rows ).

this database is growing daily and I would like to know what is the best approach of this? 

I can't think in something smart. I don't think a backup and delete all rows will be the best approach. then if we need those rows, we restore the backup.

Is there something like partitioning to be used here? 
there are 2 tables with this "problem". how can I get rid of these 2 tables, but maintaining these tables secured?
You basically have a couple of options, but in the end if you have records, they must occupy space.

You can try to compress the data. This will reduce disk space, but has side effects (it can make queries slower, for example). And it may not be available in your version of SQL. If I remember correctly, in SQL 2008 r2, compression requires Enterprise Edition. Brent Ozar's website talks a bit about some other side effects.

Otherwise, the best you can do is either move the data to cheaper (slower) disks via partitions and/or file groups or work out a retention policy that lets you purge data past a certain age (possibly after exporting it?)