since they are on month to month, just give them notice. That's most likely the quickest way to get them out anyway.
If you evict them you will win. The lease trumps their claim that that pets were negotiable. Negotiable simply means it would be considered. It does not mean you would have allowed it, so they are hooped on that issue.
However, if you evict it is likely going to cost you several hundred dollars and it could take just as much time. Save yourself the stress, give them their termination notice at the appropriate time, warn them that if there is any damage such as dog urine in the carpets, residual dog odours etc. that you will be taking it out of their deposit to remediate the problem and that if the deposit doesn't cover your expenses you will not hesitate to sue them small claims court.
At check out time, take pictures, have an impartial witness present etc.