To delete inactive subscribers, you can create two segments: one for those who haven't opened their emails (campaigns) and one for those who haven't clicked the links you've been sending, these would be the inactive subscribers in your account which you can choose to delete
Here's how to create that segment:
1. Go to Segments
2. Select "Create New Segment"
3. Give your segment a name
4. Select the list to which the segment must apply to
5. Select which conditions apply
6. Click on "Add New Rule"
From the first "Match By" dropdown, you would need to select "Event". Then from the second "Match By" dropdown you need to select "Opened" and select either any campaigns, or a particular campaign, and then select "at most : 0 times".
Then, you can save the segment. This is for non openers.
Same process for the non-clickers, but instead of opened, you would need to select 'Clicked' from the rules dropdown.
Next, go to List Subscribers Actions > Remove Subscribers > on the rules dropdown for subscribers select the subscribers segment you want to delete, then click Remove Subscribers.