Yes, absolutely. That is what thin provisioning is all about!
If you are using thin provisioning, you need to have a contingency plan in place to purchase and add more storage when your overall utilization passes a pre-determined threshold (e.g. 80%), and you need to have a monitoring system in place to ensure that you don't miss the moment. You also need to be sure that you are protected against situations wherein a single VM could begin to consume disk space explosively (e.g. Exchange or MSSQL transaction logs). For example, if you've decided that your threshold for buying more storage is 200GB of free space in your thin provisioned storage pool, it would be beyond foolish to thin provision >=200GB of extra space on any virtual server drive, even if you currently have terabytes of free space available.
If the above is too much for your organization (e.g., perhaps you cannot be entirely confident that an acquisition of additional storage would be approved when it is needed), then thin provisioning may not be an appropriate choice.
If you have already thin provisioned more space than you actually have, the first step to "undo" thin provisioning would be to reduce the sizes of the allocated partitions within the virtual disks such that, collectively, their sizes no longer exceed your physical storage limits. Then you would be able to convert to fixed-size virtual disks.
Note that converting to a fixed-size virtual disk is both time-consuming and potentially unnecessary: even if the virtual disks themselves are still technically thin provisioned in VMware, the VMs will never use unpartitioned space. For example, if you thin provisioned a 100GB virtual disk for a virtual machine that is only using 20GB of a 100GB partition, you could shrink the allocated partition within the virtual disk to 30GB. Under no circumstances would the VM use more than 30GB: the VM will still think it has a larger disk, but the unallocated space will never be used.