Any question answered...

The only way of doing it is to have someone do a proper job of sealing all the holes--mice are fairly easy to seal out. Do a thorough job once and it'll take a long time for them to reappear. All that pest control people will do is to lay poison, come back to check it, and declare the mice dead if they've touched the poison, only for the vermin (I mean the mice) to appear again in short order. (Disclaimer: I'm sure that pest control people sometimes do more, but I had them around at various times over the years and their work never did any good. I'm speaking strictly from that limited experience.)

They're also a good sign, in a way, as their presence means that you don't have rats. :)

You'd be amazed at how small a hole a mouse can get through- at quite high speed.

When I worked in the All Bar One on Regent Street the mice could jump through the air bricks in order to travel from room to room, with no appreciable drop in pace.

They may not look like it, but they can happily go through a hole the diameter of a standard biro.