You are not able to delete a contact out of a dynamic audience as this does a search of criteria you have selected. For example. Your Dynamic Audience was built to look for all men who own a 3 bedroom home.
You are able to remove contacts from a Static Audience. Here is how:
Choose Marketing & Autopilot from the left menu which is highlighted in magenta below
Choose Audiences
Search for the audience by entering the audience name or a portion of it into the search bar and click the magnifying glass
Choose the Audience by clicking on the name
Search for the contacts in two ways:
Enter their name in the search bar, click the magnifying glass
Perform a quick search to pull all contacts that fit the criteria you no longer want to include:
Delete the contact you wish to remove by clicking the delete button on the right side: