I haven't pruned my Annabelle to discourage growth, but I'd prune her with gusto if she blocked some other lovely. Pretty sure you can prune Annabelle with no problem.

Annabelle has a suckering habit and every year your plant will increase.
Cutting new growth wouldn't do any good since it will reproduce it later in a summer.
Basicly you have two options,
a) in a spring dig it out and inevitably you'll end up with a several small pieces and the main mother plant.
Use small pieces somewhere else or give to the friends and plant the large one in the same hole.
b) in a spring try to cut with a sharp shovel some chunks of it on a periphery of the clump. Sounds easier than the digging the whole plant, but in reality it works only if you cut it to 6-8" anyway, since you'll damage a lot of inner stems by working with a plant in the ground.

If I do dig it up in the spring, and make the "main mother plant" smaller and then replant, will it then perform poorly later in the summer?

How soon in the spring should I do this? Before leaves start appearing?

"will it then perform poorly later in the summer? "

Absolutely not, if you'll do it as early as possible, preferably before leaves start appearing i.e. in a dormant stage.

Like said above, you can prune to the ground if they are blocking something - won't hurt the plant. You can also try to dig or hack some of it away - which would be the only way to really reduce it's size since it does sucker.