They can, of course, pay whatever early term fee is provided for in the contract. However, early terminations don't normally include the remaining revenue due during the term. For example, lets say its a 10 year deal at 1M per year. In year 3 Fedex Early terms the deal for whatever reason. Fedex does not pay 7M which is the remaining term, but maybe just 1M in early term fees which would cover the cost of signage removal and web page updates and the like to remove FedEx name. So, yes, they can pay whatever early term fees and get out depending on the contract negotiated, but that doesn't mean there isn't a loss of revenue for the Redskins.
They can also advise they won't be renewing. And, i am quite sure the contract goes both ways - These companies do not sign away all rights - there are some termination clauses in the contract that will be enough to argue. But, and here is the big but, they can make a large enough stink and create enough pressure that Dan will just let them out of the contract with a minimum penalty. As with the contracts I deal with, if a client doesn't want to be with us, then we won't force them. that just creates a very bad relationship. I just need to recoup any costs incurred. But, the point remains, when the big money companies start pressuring like this, things tend to change.
As to Pepsi...they can absolutely add pressure whether it is local serving rights as mentioned above. However, you keep going to the contract...contracts don't cover everything under the sun. There may not be direct contracted revenue at stake, but if sufficient large companies start to 'boycot' or create sufficient pressure, it will impact the team. Now, let me ask this, if Pepsi didn't think they have some leverage, why would they have reached to Snyder and throw their 2 cents in?