At several organizations I have been at, we had situations where Sales Reps would steal other Sales Reps accounts by reassigning them to themselves.  The tricky part of this is that in a larger organization, there can be instances where the Account should be reassigned or Reps work out a "trade" between each other regarding the account.

One way I deal with this is by creating a Validation Rule on the Account level that allows the Account Owner to be the only person able to change the Ownership of the Account.

Here's the Error Message first so it's easy to understand the definition of the rule:
"Only the Account Owner can change the Account Owner."

​Here's the formula:
ISCHANGED( OwnerId )
&&
PRIORVALUE( OwnerId ) <> $User.Id
&&
$Profile.Name = 'Sales Standard User'
End Formula

The above includes the edition of a profile name but you can write other options as part of this formula.  This simple validation rule has helped to reduce tracking ownership history and emails / messages to the team to change the account owner.  When asked to change the account, I typically just default to "Ask the Account Owner to switch it to you."  It gives autonomy to the Account Owners to manage their business and accounts.

Here is a validation rule on the opportunity that is critical to making sure all opportunity close dates are current and up to date!  

I'll start with the error message so you can see the intent:
"Open Opportunities cannot have a Close Date in the past. Closed Opps cannot have a Close Date in the future. Please update your Close Date. Thanks!"

The goal here is to make sure that Close Dates are always current for open opps (meaning they can't be in the past!) and Closed Opps cannot have a Close Date in the future (you've seen it before, an Opp Closed Lost with a Close Date 2 months from now... that's just bad data integrity!  

Here's the rule to keep it clean!
Formula:
OR(

AND(
CloseDate < TODAY()- 3,
IsClosed <> TRUE
)
,
AND(
OR(
TEXT(StageName) = "Closed Lost",
TEXT(StageName) = "Closed WON"
),

CloseDate > Today()
)
)
End of Formula

Now, you'll notice that the rule is "CloseDate < TODAY()-3".  I did that to give some flexibility to the sales team as they might be working a deal and need to make changes or waiting for something on a Sunday and need an approval so I wanted to be a bit forgiving.

But otherwise this will work well to keep those Close Dates up to date as a Rep cannot save their Opp unless the Close Date is adjusted to meet the rules.