Is there a way to get rid of completed tasks (from Ethernia), or do I have to keep buying slots?

Also, I don't think I got the rewards from the tasks, so how do I claim those?

click the tick in the bottom-right corner of the task's box- with incomplete tasks it's a red "cancel task" cross

I completed them all - the "x" is now a check mark, but they didn't go away.

Edit: I realized its the same mechanism, thanks for your help!