How to protect a signal from output short circuit?
If you are making a signal generator, then the 50 ohms in the output is a good start. With a 20Vp-p sinewave and the output shorted the resistor will disipate about 1W so firstly choose this resistor to have the power rating but, what op-amp operating at maybe 30MHz can deliver 1W? Probably none so it might be worth adding a push-pull stage made from PNP and NPN emitter followers.

Because they are emitter followers and, because they can be biased (with a little care) to be on the verge of conduction (it's called class AB) taking the op-amp feedback from the output of the push-pull circuit may work OK. Be aware that it can be easy to cause ringing and oscillations so layout is a little critical but methinks you have learned this so far (did you relay the DDS and get it working better?).

The transistors should have an \$H_{FE}\$ that remains above 1 at 1GHz or higher.

Basically, my idea is to build a little power into the final drive stage and use a +1W 50 ohm resistor on the output. Be also aware that the full 20Vp-p may not be attainable using this method - maybe you can increase the power supply levels a little. Also, your choice of op-amp is worth disclosing.