The Ripple protocol requires that each Ripple wallet holds a small amount of XRP at all times. This is called reserve and its purpose is spam protection.
The main reserve requirements are:
20 XRP per address (always required - cannot be withdrawn),
5 XRP per each trustline (until disabled),
5 XRP per each open order (until executed or canceled).
A wallet has an XRP balance of 50 XRP.
It has ETH and EUR trustlines enabled.
It has one open order.
Available balance is
50 XRP - 20 XRP (base reserve) - 2 x 5 XRP (trustline reserve) - 5 XRP (open order) = 15 XRP.
Trustlines can be disabled in order to make additional XRP available. How to disable a trustline?
In EXCHANGE, the reserve is automatically set to 50 XRP as a security feature and only occurs if you click on "All available". If you input the numbers manually, you can exchange to the minimum available XRP balance.
You can find more details about XRP reserve here.
Deleting a Ripple wallet from your GateHub account will not make the 20 XRP reserve available.
Ripple wallet balances cannot be consolidated. Each active Ripple wallet must hold at least 20 XRP."Once created, an account exists in the XRP Ledger's data tree forever." More information available here.