Hi, IceClan. This is pretty simple but it is too late for this winter, probably. Chimney screen caps are available, work well and are pretty cheap. I bought one last year for $20 and installed it in about half an hour. They keep the rain out also.
There are 2 possible problems with them. The first is getting up on the roof to install. You certainly don't want to do this when you have snow and ice on the roof. The second is that bats can get through some pretty small openings. You might want to get some hardware cloth and soft wire when you get the cap and cut the cloth and wire it in place to seal the small opening that will remain with the "one size fits all" cap. And you might want to check to be sure that the bats aren't nesting in your attic and seal any holes there also.
The good news is that when you don't get eaten up in the summertime with bugs that you can thank the bats. On balance, they are nice to have around but I understand that you don't want them moving in with you.