How do I set a limit on using margin?

When you open a position in a Future contract funds are set aside to meet the future contract's margin requirement. Cash is added and subtracted from your account for the running P&L of the open position in the currency the contract is denominated in. Once the contract is closed the margin is released and the P&L for the trade remains in that particular currency. Please note, IBKR will not convert the residual non-base currency from a closed trade so it is in your best interest to monitor your account's cash balances in your Account Window.  …

A Short Sell order will be accepted and can possibly trade if you have a margin type account and there is sufficient Available Funds to meet any change in the account's margin requirements. If If you do not want the ability to sell short, you would need to have a Cash type account as Cash accounts cannot borrow shares of stock to naked short sell, be short naked Call options or short non-cash secured Put options. Cash accounts are restricted from holding a short balance in any currency as this would also constitute a loan from IBKR…

There is no feature that allows one to set a specific "Risk" limit for an IBKR account but clients do have tools available to help manage the risk of open positions.

The Account Window displays real time account values which allows one to monitor the account's margin compliance…

A long or short currency balance does not offset a margin requirement for a Future contract. Currency can be converted and traded in your IBKR Universal account's Securities segment while Futures contracts are held in your IBKR Universal account's Commodities segment. You will need to have enough Available Funds to meet the calculated margin requirement for any open Futures contracts in addition to any calculated margin requirement for an open Forex currency balance held in the account. …