Yes, you don't need to use the entire tube of TotalBoat Thixo adhesive once you open it. But after use, it's important to reseal the cartridge tightly. There are a couple of ways to do this.
If you dispense the Thixo through a static mixing tip, just leave the mixing tip attached. The adhesive will cure inside the tip - not inside the cartridge. Essentially, the used tip becomes the cap. Next time you need to use Thixo, remove the old mixing tip from the cartridge and attach a new, clean mixing tip. If you're not planning to use the product for some time, remove the mixing tip and replace the original Thixo cap.
If not dispensing the product through a mixing tip, simply remove the Thixo cap, dispense the desired amount of Thixo, clean the cartridge opening, and put the Thixo cap back on tightly.
Note that the Thixo cartridge mechanically meters how much of each component is dispensed, but when you first start dispensing from a new cartridge, one component may come out a little before the other one. For this reason, you may dispense a little more than you need. Just be sure to dispose of the extra material properly - do not attempt to put it back in the cartridge.