I did read the lease. No, the lease does not explicitly state that Les Alexander has the right to refuse another party from bringing in a NHL team; but if read the conditions in the lease, no owner in their right mind would agree to those terms.
Let's look at the terms.
The NHL team would get to pick dates to use the Toyota Center after the Rockets and anyother event that has reserved a date at least 9 months in advance. You do understand that most non Rocket events (WWE, Ringling Brothers, Disney on Ice, concerts, NCAA sporting events, Monster Trucks, etc.) are signed and dates reserved a year in advance? That means the NHL team would have to pick from a lot of bad days or undesirable times (i.e. plenty of Saturdays at noon).
The NHL owner also has to pay to purchase and maintain all the equipment necessary to host a hockey game. It has to pay Les Alexander every time the building has to be changed to hockey or to something else from hockey.
The hockey team gets to keep all the money from advertising it generates in the building, however, it cannot resell any permanent advertising installed by Alexander. Les Alexander owns the right to market and sell the suites in the Toyota Center. The hockey team can sell tickets to those suite holders, but if they don't buy tickets, the hockey team has no right to resell the suite.
In other words, the deal is slanted to Les Alexander to make it extremely undesirable for anyone else.