Are you the site owner or someone who has the permission to remove people (if you're not the site owner, worth checking permission hasn't been removed).
You should just be able to go to the member list in admin and click on the box next to their username, and then click the "remove member" button. It worked for me yesterday ... if you've checked the above, you might want to give more info - but generally probably a CS problem to solve, and they won't be back online for a day or so due to normal working hours and time zones.
I am having the same issue. I have removed many members from my site in the past, but now I am getting a 403 Forbidden error when trying to remove a member. If I switch to a different browser, then I can remove a few members, but then get the 403 Forbidden error again. I've pretty much run out of browsers to install and I still have inactive members to delete. Occasionally, I can also get it to work if I clear all my cookies and cache and browsing history, but it will only work a couple of times before the 403 Forbidden comes back.