This is by design and the Enhancement request # 109145 is filed for it:
Campaigns > Ability to display the campaign name without the campaign ID in the lead source list.
The alternate solution is presented below.
1. As Administrator navigate to Setup > Company > Auto-Generated Numbers.
2. In the CRM subtab > Deselect the Enable box for Campaign.
3. Click Save.
4. Navigate to Lists > Marketing > Marketing Campaigns.
5. Edit the Campaigns and remove the numbers in the ID field.
6. Click Save.
7. Edit an Opportunity record > Sales subtab.
The values in the Lead Source field now displays the name of the Marketing Campaign only.