Certified GKH Gardening Expert
There are products that can kill grass among ornamentals but you have to make sure your plant is on their list of safe plants. Since agapanthus has grassy type leaves, it may not be on the list.
Failing that, you can dig out the grass, trying very hard to get all the pieces of it.
If the agapanthus is not too widespread, you can dig up the bulbs, spray the grass with a non-selective herbicide, then replant.