STDs should really only be sent to the subset of possible guests who represent the select few without whom you day would not be complete (or who live on the other side of the planet and need extra lead time to get there at all). So those who receive them can reasonably assume that they will be invited because you really want them there
Even if you've chosen to spread your STDs more widely, they still go only to those who you will be inviting.
(You can probably tell that I think they're an unnecessary cost - because you can just tell the people who fall in to the groups where extra advance warning is useful without buying in special stationery - and liberal use of them can lead to messes such as this)
OP: you liked them well enough not just to invite them, but to give formal written notice of the impending invitation. So getting out of it is essentially the same as rescinding an invitation.
So either decide that - I liked them well enough before and may do so again by the time the Day comes round, and refuse to let their presence rile you.
Or you have to go public round the workplace that you are scaling back the wedding, and essentially cancel the attendance of all colleagues.
Which leaves the residual problem of colleagues you like. But are they real friends, or just people you see sometimes away from work? If real friends, you can invite them quietly later on, and rely on them not to cause difficulties for you at work. If it's just occasional socialising, then I'd have a think about how important their presence is