Bridge specific topic or except bridge topic with ros2 ros1_bridge

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If you dig into the ros1_bridge source code you will find the answer.
dynamic_bridge has the following available flags that are of interest in this case:

--bridge-all-topics: Bridge all topics in both directions, whether or not there is a matching subscriber.
--bridge-all-1to2-topics: Bridge all ROS 1 topics to ROS 2, whether or not there is a matching subscriber.
--bridge-all-2to1-topics: Bridge all ROS 2 topics to ROS 1, whether or not there is a matching subscriber.

Regarding your question for bridging specific topics you should look at the parameter_bridge:

bridge all topics listed in a ROS 1 parameter. The parameter needs to be an array and each item needs to be a dictionary with the following keys; 
topic: the name of the topic to bridge
type: the type of the topic to bridge
queue_size: the queue size to use (default: 100)

or static_bridge which gives an example implementation of bridging one topic.